PART II.

THOUGHTS TOWARDS A THEORY OF THE DEVELOPMENT OF ORGANISMS.[17]

Now that criticism of the germplasm theory has given us a bias in the right direction, it is necessary to map out more clearly the path along which solution of the problem may be sought. In general terms, our problem is the necessary origin from an egg, always of the same organism, with its manifold characters, and the explanation must avoid the attribution to the egg of characters foreign to its nature as a cell. This is the more necessary as Weismann objects to the supposition that cell-division is doubling, holding that the supposition allows neither an explanation, nor even the beginning of an explanation, of the differences that arise among cells while the differentiation of the body occurs. 'Any explanation must in the first place account for this differentiation,' says Weismann (_Germplasm_, p. 224); 'that is to say, the diversity which always exists amongst these cells and groups of cells arising from the ovum must be referred to some definite principle. In fact, no one could even look at it as giving a partial solution of the problem, if differentiation is supposed to be due to that part alone of the germplasm becoming active which is required for the production of the cell or organ under consideration. But the higher we ascend in the organic world, the more limited does the power of producing the whole from separate cells become, and the more do the numerous and varied differentiations of the soma claim our attention and require an explanation in the first instance. The presence of idioplasm in all parts containing the primary constituents does not help us in this respect.'

With this I cannot agree. Naturally, Naegeli, De Vries, Driesch and I assume that, of the many rudiments present in every cell, only some come to activity in each special case, and that the selection of those that become active is due to causes arising in the course of development. Our conception of the nature of these causes, and of their place of origin, is diametrically opposed to Weismann's.

Weismann would make the causes of this orderly development of the rudiments reside in the germplasm itself; for he considers that to be not only the material but the motive force of the course of development. According to him, every cell _must_ have become what it is, because it was provided only with the definite rudiments assigned it beforehand, according to the plan of the development of the germplasm.

On the other hand, we regard the development of the rudiments as depending upon motive forces or causes that are external to the germplasm of the ovum, but that none the less arise in orderly sequence throughout the course of the development. The causes we recognise are first, the continual changes in mutual relations that the cells undergo as they increase in number by division, and second, the influence of surrounding things upon the organism.

One may group together as _centrifugal causes_ of the process of development the characters of the fertilised cells and the interrelations between the products of their divisions, and distinguish them from the _centripetal causes_, or motive forces that are provided by the action of surrounding things. None the less, it must be borne in mind that there is no sharp distinction between centrifugal and centripetal forces. On page 86 I showed how what is external in one stage of the process becomes internal in the succeeding stage. The external constantly is becoming internal, and the sum of the internal factors increases only at the expense of external factors.

From the physiological point of view I regard the divergent differentiation of cells as a reaction of the organic material to unlike impelling forces--that is, to factors shown by experimental physiology to be actually present and to rule the building up of the organism. 'It were superfluous to detail,' as Naegeli says, 'how continually other forces external to the idioplasm, but belonging to the individual, influence the idioplasm; every cell, indeed, as it grows and divides, takes up a definite place in the growing whole, and finds itself in a peculiar combination of conditions of organisation.' 'Not only influences within the individual affect the idioplasm, as that may be altered by external influences, and so may be forced to grow in a new direction.' 'The influence of surroundings in determining which of the rudiments contained in the idioplasm shall achieve development is shown in the following example: it depends on their nutrition whether certain trees shall bear foliage or flowers; while in an unpropitious climate many plants refuse to bear flowers at all, but content themselves with vegetative reproduction.'

This principle indicates the path along which explanation of the differentiation of cells is to be sought. Although in no single case is it yet possible to refer a known action to its appropriate cause--in other words, to show a definite stimulus producing a definite reaction upon the rudiment--this failure is not to be attributed to error in the principle. It is the natural result of the enormous difficulties besetting an attempt to understand the highly involved events of development. We can only ask whether or no our general principle is harmonious with the facts displayed in nature.

In the following pages I shall try to develop this view, taking, as formerly, a few instances. I shall now proceed further with suggestions I made in my treatise on _Old and New Theories of Development_. I start from the conception that the ovum is an organism that multiplies by division into numerous organisms like itself. I shall explain the gradual, progressive organisation of the whole organism as due to the influences upon each other of these numerous elementary organisms in each stage of the development. I cannot regard the development of any creature as a mosaic work. I hold that all the parts develop in connection with each other, the development of each part always being dependent upon the development of the whole.

The power of the egg to multiply by division is a chief and most important factor in the production of complexity during the course of development. It is only because the nuclear material, by a series of intricate, chemical changes, assimilates reserve material from the egg and oxygen from the atmosphere that it can give rise to continually increasing complexity within itself. The increase in bulk results in a cleavage into two, four, eight, and sixteen pieces, and so forth. The cleavage produces a constantly changing distribution in space of the nuclear material. The two, four, eight, and sixteen nuclei that arise by division diverge from each other and take up new positions inside the egg, in definite relations to each other. At first the particles of the egg were arranged around the fertilised nucleus, which was a single centre of force; they become grouped around as many centres of forces as there are nuclei, and so become segregated into as many cells. Clearly enough, the egg, in its single-celled condition, changes its quality in a marked degree when it becomes multicellular, even although the change has occurred by doubling division.

This, so clear in the early stages of development, continues to occur throughout the later stages of growth. The continued cell-multiplication causes not only changes of bulk, but also from time to time changes in quality; for each shape is bound up with definite conditions. When the conditions alter, the organic material, by its power of reaction, changes its shape in a corresponding fashion.

As the nature of architectural plans depends upon the properties of the wood, stone, or iron, as they must correspond with the material to be employed (_i.e._, the span of a roof, the construction of a bridge depend upon the material in shape and weight), so the nature of the organic material determines to a large extent the shapes assumed in the course of growth.

Shape in many respects appears to be a function of growth in an organic material.

A few examples will make clear this important relation. A limit is set to increase in the size of a blastosphere by the nature of the material of its walls. Its wall is a membrane, composed of one or more layers of cells; that this may preserve its curvature, a definite pressure from within must be maintained, proportioned to the cohesive force of the cells; at the same time the wall of the sphere must be able to withstand the strain and pressure put upon it by external forces. All these, and many other factors less easy to conceive, must be delicately adjusted to one another. If in any direction a definite limit be exceeded, then either the structure will be destroyed by disintegration of the component parts, or a new shape will be assumed. The latter is the event in the case of a living substance capable of reaction. The blastosphere, growing beyond its limits, folds into a cup-shaped organism. Did we know all the influences affecting the wall of the blastosphere, then we would understand the causes by which growth beyond a definite limit must result in invagination. From the occurrence of the gastrula in all the divisions of the animal kingdom, we may conclude that it is a temporary phase, inevitable in the growth of animals.

There may be noticed here a second connection between shape and organic growth, exceedingly simple in its nature, but of fundamental importance in its consequences. It may be stated in this saying: Growth always must be such as to produce the greatest possible extension of surface. The reason of this is simple, depending on the different natures of inorganic material and living organic material.

A crystal in its mother liquor grows by attracting new particles and depositing them upon its outer surface, according to the kind of crystallisation peculiar to the material of which it is composed. These particles, once crystallised, retain their position even when new layers are deposited on their outer surfaces, and remain unchanged, perhaps, like rock crystals, for thousands of years, until changed outer forces loosen the bonds that bind them.

Organised material cannot grow in this fashion; it takes up material from without, not, like the crystal, arranging it on the outer surface, but ingesting it. Protoplasm cannot become fixed in any condition without being destroyed; it exhibits perpetual interchanges with the outer world; unceasing intake and output is a necessary accompaniment of its life. 'The growth of idioplasm,' as Naegeli strikingly says, 'implies a constancy of perpetual change.'

Thus, growing protoplasm can assume only such shapes as allow it to remain in constant touch with the outer world. A cubical or spherical mass of cells could not grow by the formation of new layers of cells on the outside, for these layers would deprive the centrally placed masses of cells of their conditions of existence. Similarly, an extended membrane of cells or an epithelial layer cannot add indefinitely to its thickness, else would the cells furthest removed from the outside be injured in their relations to surrounding things. To satisfy its essential conditions, protoplasm can grow only with a proportionate extension of its external surfaces. This is secured by the cells becoming arranged in threads and membranes, and its result is that the threads by branching, and the membranes by folding, produce structures whose complexity increases with growth.

This conception that the shape of growing organisms is in many respects the necessary consequence of the specific characters with which protoplasm is endowed, explains the great contrast between animals and plants in their general organisation. The contrast is the result of the difference between animal and plant metabolism, and between the ways in which animals and plants obtain their food. Plant cells elaborate protoplasm from the carbonic acid of the air, water, and easily diffusible solutions of salts, obtained from the sea or from the soil. For the chemical work of combining these, they require the active energy of sunlight. We can now see the chief requirements to which the constitution and arrangement of the cells in a multicellular plant must be adapted. Plant cells may become clothed in a thick membrane, as that would prove no hindrance to the passage of gases and easily diffusible salts; but they must be arranged so as to present the greatest possible surface to the surrounding media (_i.e._, to the soil and the water, the air and the sunlight) whence is drawn their supply of matter and force. The cells must turn a broad face to the outside; this they do by becoming arranged in branching rows, or in leaf-shaped flattened organs. That they may suck up water and salts from the soil, the cells are arranged as a highly branched system of roots, covered with delicate hairs, and penetrating the soil in every direction. To inhale the carbonic acid from the air, and to be subjected to the influence of sunlight, the aerial part of the plant stretches out its branches towards the light, and becomes folded into the flat leaves, the structure of which reveals a suitability for assimilation. Thus the whole architecture of a plant is superficial and visible; internal differentiation into organs and tissues either is wanting, or, compared with animals, is very scanty. It is only in the higher plants that the internal fibro-vascular tissues appear; these serve a double purpose: they act as channels along which the sap passes, so bringing together the different materials absorbed by roots and leaves; and they have the mechanical function of strengthening the stem and branches. The different mode of nutrition of animals results in a totally different structural plan. Animal cells absorb material that is already organised, and that they may do so their cells are either quite naked, so affording an easy passage for solid particles, or they are clothed only by a thin membrane, through which solutions of slightly diffusible, organic colloids may pass. Therefore, unlike plants, multicellular animals display a compact structure with internal organs adapted to the different conditions which result from the method of nutrition peculiar to animals. A unicellular animal takes organic particles bodily into its protoplasm, and forming around them temporary cavities known as food vacuoles, treats them chemically. The multicellular animal has become shaped so as to enclose a space within its body into which solid organic food-particles are carried and digested, thereafter, in a state of solution, to be shared by the single cells lining the cavity. In this way the animal body does not require so close a relation with the medium surrounding it; its food, the first requirement of an organism, is distributed to it from inside outwards. In its further complication the animal organisation proceeds along the same lines. The system of internal hollows becomes more complicated by the specialisation of secreting surfaces, and by the formation of an alimentary canal, and of a body cavity separate from the alimentary canal.

In plants, it is the external surface that is increased as much as possible. In animals, in obedience to their different requirements, increase takes place in the internal surface. The specialisation of plants displays itself in organs externally visible--in leaves, twigs, flowers, and tendrils. The specialisation of animals is concealed within the body, for the internal surface is the starting-point for the formation of the organs and tissues.

Comparative embryology shows that, however varied the forms and functions of the numerous animal organs may be, the method of their development is remarkably similar. There are required only the slightest variations of a few simple general laws. For these I may refer readers to a series of special investigations (_Studies on the Germ-layer Theory_, Oscar and Richard Hertwig), and to the fourth chapter of my Embryology, 'General Discussion of the Principles of Development.'

In these works and in the foregoing pages I have tried to show that the multiplication of the egg-cell by division is itself a source of increasing complexity and an active principle in the determination of form, since the products of the division unite to form a higher unity. But in another way the multiplication of cells leads to differentiation among the cells arising from the egg. Although each of these resembles the parent egg, from which they arose by doubling division, yet they differ from it in one point: they are no longer a whole, but have become the subordinate parts of a higher unity, that is, of a higher organism. A cell that is no longer a whole, but the part of a whole, has entered upon reciprocal relations with other cells, and in the functions of its life is limited by these others and by the whole. The further this is carried the more the cell falls short of its independence as an elementary organism, and appears only as a part with its functions subordinate and in dependence upon the whole.[18]

Although from the point of view of morphology it has become more and more imperative to regard the cell as the unit of the higher organism, still, from the physiological point of view the higher organisms must be regarded as masses of material acting as wholes, and composed of several grades of structural parts, subordinate in function to the whole, and displaying only a limited division of capacities. And so the cell theory, according to which the cell was exalted unduly as the unit of life, the centre of life, the elementary organism, must take limitation and correction from these wider views. This has already been insisted upon by many physiologists of insight--for instance, by Naegeli (see p. 30), by Sachs, and by Voechting.

'Cell formation,' declares Sachs (_Physiology of Plants_, p. 73), 'is a phenomenon very general, it is true, in organic life, but still only of secondary significance; at all events, it is merely one of the numerous expressions of the formative forces which reside in all matter, in the highest degree, however, in organic substance.' 'Essentially, every plant, however highly organized, is a continuous mass of protoplasm, surrounded externally by a cell wall and penetrated internally by numerous transverse and longitudinal partitions.'

My conception receives strong support from the way in which Voechting set forth the relations of the cell to the whole:

'Is the circumstance that a cell, separated from the organism, is able to survive and build up the whole again a proof of the independent life of the cells while in the organism? I believe it to be only a proof that the life of the organism is always dependent upon the cell, that the life is inherent in the cell, and that the life of a compound organism is merely the resultant of the vital phenomena of its single cells; but by no means that the cell when isolated displays the same functions as while it is a part of the organism. The cell while in the organism and the cell separated from the organism and self-sufficing, are quite different. We must regard the functions of a cell that is part of an organism, disregarding external influences, as determined by the whole organism, and only by the cell itself, in so far as that forms a greater or less part of the whole organism. When not part of an organism, the cell is independent, and entirely determines its own function. Nowhere is it easier than in this case to confuse possibilities with facts, and nowhere is the confusion more fatal. From a morphological point of view, one may confidently regard the cell as an individual; but it must be borne in mind that an abstraction has been made. Physiologically considered, the cell is an individual only when it is isolated from a complex and is independent; of this no abstraction can be made.'

According to the conception I have been explaining, cells merge their independent individuality in that of the whole, and so the force that directs their ultimate development, and that leads to their appropriate elaboration, cannot be within them, cannot reside in special groups of determinants, in the sense of Weismann. It is given by the relations in which the cells come to stand to the whole organism and to the various parts of the organism, and, on the other hand, to surrounding things. Naturally, such relations differ with the place or position occupied by cells in the whole organism, and in this way there come to be innumerable conditions making for diverging directions of development, for division of labour, and for dissimilar, histological differentiation. The part played by a cell, as Voechting puts it, will depend upon the position it comes to assume in the whole living unit. To use an expression of Driesch's, dissimilar differentiation of cells is a 'function of position.' Such a conception my brother and I, in our _Studies on the Germ-layer Theory_, sought to establish clearly by many examples from the histology of the coelenterates and of higher animals; such a conception for long has been clearly expressed in physiological botany.

The simpler nature of plants in structure and function makes it easy to conduct experimental observations upon this point.

I have already described how either side of the prothallus of a fern may be made to produce male or female organs, according as it is kept in the light or in the dark. Similarly, taking a willow slip, roots may be made to appear at one end by moisture and darkness, while they will not appear on the end kept in the light.

The experiments of botanists and of fruit-growers show that young buds and the rudiments of roots are indifferent structures, the further growth of which depends entirely upon the conditions in which they are placed. 'One and the same bud may grow to a long or short vegetative shoot, to a floral shoot, to a thorn, or may remain undeveloped. The same root rudiment may grow to a main tap-root or may form a secondary lateral root. The conditions that determine the mode in which these structures will develop are quite within the power of the experimenter. We have shown already and could show further, that he is able to determine the mode of growth by cutting, bending, tying in a horizontal position, and so forth: For such reasons, Voechting describes plants as masses of tissue, practically plastic, and which may be moulded at the discretion of the investigator. 'For instance, in the case of _Prunus spinosa_, a branch may be produced in place of a thorn by cutting a growing shoot at the proper height, in spring. The buds below the point where the cut was made turn to shoots like the rest of the plant and complete the interrupted growth, while on an uncut stem they would have grown to thorns. Thus, the rudiment of a thorn has been changed to that of a shoot' (Voechting).

Although it is more difficult to carry out experiments upon animals, some good instances are known. If a piece cut from the stem of _Antennularia_ (a hydroid polyp) be placed vertically, in a short time new branches and new 'roots' spring from it. In this case, again, the position of the new growths is determined by the relation in which the stem is placed to gravity. 'The tentacles arise only at the end turned towards the zenith; the "roots" from the parts directed towards the ground' (Loeb).

A similar example may be taken from among vertebrates. The notochord arises from a set of cells which are in close relation with the fused tips of the blastopore. By exposing developing frog's eggs to abnormal conditions, I was able, in some cases, to produce a hypertrophy of one of the lips of the blastopore. When fusion of the lips took place the normal lip united with the rim of the protruding hypertrophied lip. As a result of this the notochord and the nerve plate came to arise, not from the usual set of cells, but from those cells that, by the abnormal condition, had come to lie in the place for the notochord. The protruding cells, which normally would have developed into notochord and nerve plate, grew into a simple fold of the external skin.

Moreover, it is well known in pathology that mucous membranes may lose their proper character and assume the qualities and aspect of the external skin, when, as in cases of prolapse, fistula, etc., they have been exposed for some time to the air.

The relations of different parts to each other and to the whole are known as correlations. Correlation exists in all the stages of the development of an organism, sometimes in one way, sometimes in another. One must note very carefully that Weismann's doctrine of determinants, according to which all that happens in development follows a prearranged plan, is entirely in opposition to this correlative character of the changes that occur during development.

Here I shall give a few quotations from botanical and zoological writers:

'If the stem of a plant be cut so that it retains its roots, but is deprived of leaves and shoots, then the adventitious buds will produce new leaves and shoots. If, however, the stem be cut so as to deprive it of roots, then the same cells that in the other case produced leaves and shoots will now produce roots. Precisely the same occurs with a piece of the root. In fact, it appears as if the idioplasm knew what parts of the plant were wanting, and what it must do to restore the integrity and vital capacity of the individual.' 'The idioplasm in the remaining part of a plant must be affected when an important part has been removed, because the idioplasm of the lost part is no longer capable of having influence.' 'It is clear enough that necessity acts as a stimulus, and that each definite need calls into existence the appropriate reaction.'

These are Naegeli's views, and they have been elaborated by Pflueger in his important treatise on _The Teleological Mechanism of Living Nature_ (1877).

Voechting writes in similar fashion:

'In a tree that is growing under normal conditions, without being subjected to injury, all the organs appear in definite relation to each other: so many leaves correspond to a definite number of twigs and branches. These spring from a stem of proportionate thickness, and the stem passes into a definitely proportioned tap-root, from which arise a due array of lateral roots. In normal conditions all these organs are in equilibrium. An apple-tree, growing on the line where tilled garden ground meets a lawn, grows more vigorously on the side towards the garden. If one of the roots of an apple-tree with three main roots and three branches be amputated, then the corresponding branch will lag behind in growth, although it may not absolutely perish.' 'The equilibrium varies according to the specific nature of the tree. It is shown in one way in the oak, in another in the beech, and is different in the varieties of a species.'

Finally, consider this statement from Goebel's _Treatise on the Morphology and Physiology of the Leaf_: 'The fact that lateral buds do not develop while the axial bud is still growing vigorously depends upon the relation between the two. That I denote as correlation of growth.'

The dependence of parts upon each other, and upon the whole, is specially clear and instructive in cases where different plant individuals are united by budding or grafting. To limit the growth of a tree, and to induce it to become dwarfed, it is necessary only to graft it upon a nearly allied but dwarf variety. When a pear-tree is grafted upon the quince, which is characterized by its dwarf-like growth, the vegetative growth of the pear is reduced exceedingly. It produces shorter and weaker shoots; all the dwarf varieties of the pear employed as wall fruits, or growing into the little pyramids spoken of in the trade as 'cordon'-trees and potting-trees, could not have been produced unless the gardener had had the quince as a natural dwarf stock (Voechting). With the dwarfing is associated a freer and earlier production of fruit. Other kinds of fruit-trees, apples, apricots, and so forth, show the same course.

'The capacity to withstand external influences and the duration of life may be altered in the same way. The pistachio (_Pistazia vera_), cultivated in Frankfort, which is destroyed by a temperature lower than 7.5 degrees of frost, will survive 12.5 degrees if it has been grafted upon _P. terebinthus_. Moreover, when it is grown from a seedling, it may reach the age of 150 years; but when it has been grafted upon _P. terebinthus_ its length of life is increased to 200 years; while, grafted on _P. lentiscus_, it reaches only about 40 years' (Voechting).

Voechting's experiments upon beetroot are still more characteristic. 'The stem of a beet plant that bore young buds gave rise to vegetative shoots when it was united with a young, still growing root, but to a blossoming stem when it had been grafted, in spring, upon an old root.'

Similarly, animal growth is correlative in all its stages. When a muscle becomes unusually large it sets up corresponding correlations of growth in many other parts of the body. The bloodvessels and nerves supplying it become larger, and the increase in the nerves leads to corresponding increase in the nerve centres. The tendons of origin and of insertion, and the parts of the skeleton to which these are attached, must react to the increased size of the muscle by growing larger; in fact for all the parts of the animal body the conclusions which Naegeli and other physiologists drew from plants are applicable. All the different elements of the body are in definite and intimate touch with each other.

This is shown most beautifully and clearly in the extraordinarily interesting phenomena called dimorphism and polymorphism. These seem to me to show how very different final results may grow from identical rudiments, if these, in early stages of development, be subjected to different external influences.

Finally, I have a little to say about the sexual dimorphism that occurs so generally in the animal kingdom.

Nearly all kinds of animals appear as male or as females. These differ from each other not only in that they produce eggs or spermatozoa, but frequently in a number of more or less striking characters affecting different parts of the body, and known as secondary sexual characters. In fact, the difference between the sexes may be so great that a systematic naturalist, unacquainted with the mode of development of the creatures, might place them in different species, genera, or even families, on account of the striking differences in external characters.

As an instance, take _Bonellia_, a gephyrean, the strange case of which has been remarked upon by Hensen and by Weismann. The male is about a hundred times smaller than the female, in the respiratory chamber of which it lives as a kind of parasite, and appears, so far as outward shape goes, more like a turbellarian than a gephyrean. None the less, male and female are alike not only while they are in the egg, but as larvae, and it is only towards the period of sexual maturity that the great difference between them begins to appear. So also is it with the dwarf males of the cirripedes.

Males and females, whether they be more or less unlike, arise from the same germinal material. The germinal material itself is sexless; that is to say, there is not a male and a female germinal material. The phenomena of inheritance in the sexual generation of hybrids show this clearly. Characters appropriate both to males and to females are transmitted either by eggs or by spermatozoa. In parthenogenetic animals both male and female individuals appear at definite times from eggs produced without sexual commerce. Whether the male or the female forms be produced depends, not upon any difference in the germinal material, but on the external influences, just as external influences determine whether the bud on a twig shall give rise to a vegetative or to a flowering shoot, to a thorn or to a stem. The influence of food, of temperature, or probably of other agencies, determines in which direction the germinal material shall grow.

The experiments of a distinguished French investigator, M. Maupas, on the determination of sex in _Hydatina senta_, a rotifer, have given striking results.

In _Hydatina_, under normal conditions the eggs of certain individuals give rise always to males, of others always to females. By raising or lowering the temperature at the time when the eggs are being formed in the germaria of the young females, the experimenter is able to determine whether these eggs shall give rise to males or to females. After that early time the character of the egg cannot be altered by food, light, or temperature.

In one experiment, in which five females not yet fully grown were kept in a room at the temperature of 26 to 28 degrees centigrade, Maupas found that, of 104 eggs only 3 per cent. gave rise to females, while in the case of other five young females of the same brood, but kept in a cold chamber at a temperature of 14 to 15 degrees centigrade, 95 per cent. of females were produced. In another experiment, young animals were kept for a few days in the cold, and then, until death, in a higher temperature. Of the eggs produced while in the cold, 75 per cent. produced females, of those deposited in the warmth, 81 per cent. became males.

With these results may be compared what happens with many plants. Melons and cucumbers, which produce on the same stem both male and female flowers, bear only male flowers in high temperatures, only female flowers when subjected to cold and damp.

In the case of many insects in which parthenogenesis occurs, the determination of sex depends upon fertilisation. Thus, among bees, unfertilised eggs give rise to drones, fertilised eggs to females.

Sexual dimorphism in still another way reveals the intimate interactions existing between all the parts of an organism in every stage of development. It is well known, for instance, that among animals the early removal or destruction of the sexual organs hinders the development of the secondary sexual characters, or even may occasion the appearance of the characters of the other sex. Old hens become cock-feathered; human eunuchs have the high-pitched voice and the peculiarities of the larynx found in women.

As much as sexual dimorphism, the phenomena of polymorphism show the enormous influence exerted by external forces upon correlated variation of the parts during development, and in this way upon the final structure.

In the question of polymorphism it is worth while to discuss at some length the extreme polymorphism exhibited in the case of some of the colonial animals--first, because the matter has recently occasioned an important controversy between Herbert Spencer and Weismann; and, secondly, because the discussion will serve to make still more clear the difference between my views and those of Weismann upon the nature of the process of development.

Among the colonial insects there arise, in addition to males and females, sexless individuals known as neuters. These in certain cases are very different from both males and females in structure and in social instincts.

Among bees there are the queens, sexually mature females; the workers, females whose sexual organs are rudimentary, and parts of whose bodies--the stings, the wings, the hind legs, with their pollen-collecting apparatus--are peculiarly formed; and, lastly, the males, or drones.

In many of the ant and termite colonies still greater differences exist between the different sets of individuals. In addition to males and females, there are sexless workers, and these is many species are of two kinds, known as workers and soldiers. The divergences of structure among the three or four forms are shown, frequently by considerable differences in size, by the presence and absence of wings, by differences in the sense-organs, the brain, and the structure of the head. In the common ant--_Solenopsis fugax_, for instance, as Weismann quotes from Forel--the males have more than four hundred facets on their eyes, the females about two hundred, and the workers from six to nine. Many soldiers possess enormously large and heavy heads, with massive jaws, and naturally, with the appropriate muscles much enlarged.

But as workers and soldiers, on account of the rudimentary state of their sexual organs, cannot reproduce themselves, all the three or four kinds of ants in the colony must be developed from eggs deposited by the females. In this Weismann finds the most convincing proof of the omnipotence of natural selection, and, I venture to add, for the omnipotence of his doctrine of determinants.

He says (_Contemporary Review_, vol. lxiv., p. 313): 'It fortunately happens that there are animal forms which do not reproduce themselves, but are always propagated anew by parents which are unlike them. These animals, which thus cannot transmit anything, have nevertheless varied in the past, have suffered the loss of parts that were useless, and have increased and altered others; and the metamorphoses have at times been very important, demanding the variation of many parts of the body, inasmuch as many parts must adjust themselves so as to be in harmony with them.' 'None of these changes' (p. 318) 'can rest on the transmission of functional variations, as the workers do not at all, or only exceptionally, reproduce. They can thus only have arisen by a selection of the parent ants, dependent on the fact that those parents which produced the best workers had always the best prospect of the persistence of their colony. No other explanation is conceivable, and it is just because no other explanation is conceivable that it is necessary for us to accept the principle of natural selection.'

According to Weismann's conception, 'every part of the body of the ant' (_loc. cit._, p. 326) 'that is differently formed in the males, females, and workers is represented in the germplasm by three (sometimes four) corresponding determinants; but on the development of an egg never more than one of these attains to value--_i.e._, gives rise to the part of the body that is represented--and the others remain inactive.' This structure of the germplasm Weismann attributes to the operation of selection. 'For in the ant state' (_loc. cit._, p. 326) 'the barren individuals or organs are metamorphosed only by the selection of the germplasm, from which the whole state proceeds. In respect of selection, the whole state behaves as a single animal. The state is selected, not the single individuals, and the various forms behave exactly like the parts of one individual in the course of ordinary selection.'

Naturally, from the views on the germplasm theory and on the doctrine of determinants that I have expressed in this book, I cannot accept the explanation Weismann thus gives of the facts. It is true that Weismann holds his own explanation to be the only conceivable explanation. 'For there are only two possible _a priori_ explanations of adaptations for the naturalist, namely, the transmission of functional variations and natural selection' (_loc. cit._, p. 336); 'but as the first of these can be excluded' (on account of the infertility of workers and soldiers), 'only the second remains.'

But are the alternatives really only as Weismann suggests? Is there no choice left for the naturalist?

When I was reading his _All-sufficiency of Natural Selection_, kindly sent me by the author, it came into my mind that I could not accept his dilemma. For the different individuals in the insect states may be explained in a third way--in a way overlooked by Weismann. This third explanation is nothing more than the subject of all this treatise of mine. It is that, in obedience to different external influences, the same rudiments may give rise to different adult structures.

I am glad that the same answer has been made to Weismann's _All-sufficiency of Natural Selection_ by two biologists, Herbert Spencer and Emery, simultaneously with mine. Emery, a specialist upon the structure of ants, and Herbert Spencer, relying upon the investigations of several Englishmen, have sought to prove that the differences between the individuals in the colonies of ants, bees, and termites, have been slowly called into existence by the operation of external influences affecting the egg in its situation and food during development.

It has been shown fully by experiment and by observation that the fertilised eggs of the queen bee may become either workers or queens. This depends merely on the cell in the hive in which the egg is placed, and on what food the embryo is reared. In the specially large cells, known as queens' chambers, and with specially nutritious diet, they become queens. With poor food, and in smaller cells, they become workers. Even if worker larvae be supplied in time with a richer diet, they may be turned into queens.

Similarly, the differences that exist among termites and ants, as Emery shows, may be described as polymorphism due to food. The Italian zoologist, Grassi, has shown that termites have it in their power to alter the relative numbers of workers and soldiers, and to produce as many of the latter as may be required, and they are able to accelerate the sexual maturity of other individuals by supplying nourishment suitable for stimulating the maturation of the genital organs.

Emery explains this polymorphism by attributing it to the general laws of growth in the insect organism under the influence of different external stimuli. He thinks that 'the production of workers depends upon a special capacity of the germplasm to respond to the abundance or scantiness of certain nutritive materials by a greater growth of certain parts of the body, and a lesser growth of other parts. Workers' food stimulates growth in the jaws and brain, retards growth in the wings and sexual cells. Queens' food has the opposite action.' There is a correlation between retardation of the sexual glands and acceleration of the development of the head, just as in vertebrates there is a correlation between the sexual glands and the secondary sexual characters. 'The characters by which the workers differ from the queens, therefore, are not innate, but are produced secondarily.'

Quite independently, but simultaneously, Herbert Spencer has suggested the same explanation as Emery. Moreover, he has used the conditions that exist among the state-forming insects as a strong argument against Weismann's doctrine of determinants. The observations of many careful persons, such as Charles Darwin, Emery, and others, show that in many species of ants the extreme types of individuals are connected by many intermediate forms. (_Apud_ Emery, this is the case in many _Myrmicidae_, in most _Camponotidae_, and in _Azteca_.) These forms are transitional, not only in general size, but in the degree to which the genital organs have been arrested, and in the peculiarities of the jaws.

Spencer explains these transitional forms, and I agree with him, by supposing that the stoppage in food supply has taken place at different times after development has begun. ('It must happen that the stoppage of feeding will be indefinite.') Thus, the existence of transitional forms presents no difficulty on the theory of the agency of food. But how can the doctrine of determinants be applied to it? 'If he is consistent' (says Spencer, _Contemporary Review_, lxiv., p. 901), 'he must say that each of these intermediate forms of workers must have its special set of "determinants," causing its special set of modifications of organs; for he cannot assume that while perfect females and the extreme types of workers have their different sets of determinants, the intermediate types of workers have not. Hence we are introduced to the strange conclusion that, besides the markedly distinguished sets of determinants, there must be, to produce these intermediate forms, many other sets slightly distinguished from one another--a score or more kinds of germplasm, in addition to the four chief kinds. Next comes an introduction to the still stranger conclusion, that these numerous kinds of germplasm producing these numerous intermediate forms are not simply needless, but injurious--produce forms not well fitted for either of the functions discharged by the extreme forms, the implication being that natural selection has originated these disadvantageous forms. If, to escape from this necessity for suicide, Professor Weismann accepts the inference that the differences among these numerous intermediate forms are caused by arrested feeding of the larvae at different stages, then he is bound to admit that the differences between the extreme forms, and between these and perfect females, are similarly caused. But if he does this, what becomes of his hypothesis that the several castes are constitutionally distinct, and result from the operation of natural selection?'

My course of thought leaves me with little to add to this criticism by Spencer. In this case, as in many others that I have pointed out, Weismann makes his usual mistake. He incorporates in the rudiment what really are stimuli coming from external conditions during the process of development; he makes a grave confusion between the rudiment and the conditions of its development.

In my view, in these cases of polymorphism in the colonies of insects Nature exhibits a series of most important experiments, and their plain meaning is that the same germinal material, when subjected to different external influences, may produce very different final products. When from the neutral germinal material of an insect egg there is produced a male or female creature, or a worker or soldier (as this or that influence acts), the process is no other, and presents no greater difficulties, than when an experimenter, taking the young bud of a plant, according to the conditions to which he subjects it, can turn it into a vegetative or into a reproductive shoot, a thorn or a root; no different to what occurs when the investigator, cutting into a _Cerianthus_, produces a second or third mouth, surrounded by tentacles, or in the case of _Cione_ surrounded by eye-spots.

It has been shown, I think, in these pages that much of what Weismann would explain by determinants within the egg must have a cause outside the egg. The chief factors in the process of development we have found to be: (1) The multiplication of cells by division (growth as a moulding factor); (2) the relations of cells to their external environment (position in its widest sense as a factor); (3) the interrelations of the parts of a whole (cells, tissues, and organs) to one another and to the whole (correlative development). There remains to be considered the extent to which the germinal material in the egg determines the course of development of the organism. Here, before all things, it must be insisted that the individual nature of the cell determines the specific fashion in which the cell will react to the varying stimuli coming from varying conditions. The same agency produces very different results upon different organisms. These differences must be attributed to the differences in the nature (different intimate structure) of the active material.

Sachs speaks strikingly on this point (_Physiology of Plants_, p. 602): 'If the same external cause induces exactly opposite effects in the organs, the explanation of this must simply be sought in the different structure of the organs. If one organ, when illuminated from one side, becomes curved so as to be concave on the side turned towards the centre of light, while another becomes convex on that side, the cause can only lie in the internal structure of the organ. But it is just on such differences of structure that the great variety of reactions which the most different plant organs exhibit towards the same external influences depends; and, fundamentally, all that we term biology--the mode of life of organisms--depends upon the fact that different organisms react differently towards the same external influences, and these reactions differ not only qualitatively, but also quantitatively, the finest gradations existing in both cases.'

For instance, in a plant-embryo roots are produced at the lower end under the influence of the soil and of gravity. But it is upon the specific nature of the protoplasm of different kinds of plants that the special shape of the whole root system depends: whether, for instance, the root system ramifies superficially or strikes deep into the soil; whether the rootlets grow quickly or slowly; in what fashion they fork, and whether or no they form special structures like bulbs.

Thus, even from my point of view, explanation of the process of development requires the assumption of the existence of different kinds of germinal material in different kinds of organisms. These germinal substances must be possessed of an extraordinarily complex organisation, and must be able to react in specific fashion--that is to say, in a fashion different in each species--to all the slightest internal and external stimuli encountered from time to time as the organisation becomes formed by cell division.

In this sense I agree with what Naegeli says:

'The egg-cells contain all actual specific characters as truly as the adult organisms; when they exist in the condition of eggs, organisms are as distinct from each other as in the adult condition. The species is present as truly in the fowl's egg as in the fowl, and the egg of a fowl differs as much from the egg of a frog as the fowl differs from the frog. Men, rodents, ruminants, invertebrates display more or less important and outwardly visible differences in constitution; so also the sexual cells to which they give rise, since they represent the rudiments of the future adults, must be different from each other in the constitution of the rudiments, although we are not yet able to prove these differences by observation.'

In this assumption of a specific and highly-organized germinal substance with which a development begins, I agree with evolutionists; but in its details my conception is quite different from their conception. For I can ascribe to the germinal substance only such characters as are appropriate to the true nature of a cell, but I cannot ascribe to it the numerous characters that can come into existence only by the interrelations of many cells and the action of the environment.

Haacke, in his recently-published book (_Gestaltung und Vererbung)_, has expressed a doubt that my conception of development is, after all, a preformational theory. 'For preformation,' he says, 'it is not necessary to imagine that the egg contains a miniature of the adult. If only, like Hertwig, one assumes to be present in the germinal material a prearrangement of qualitatively different idioblasts, one has steered into the harbour of preformation with all sails set.'

In reply, I plead that, like Naegeli, De Vries, Driesch, and others, I have tried to blend all that is good in both theories. My theory may be called _evolutionary_, because it assumes the existence of a specific and highly-organised initial plasm as the basis of the process of development. It may be called _epigenetic_, because the rudiments grow and become elaborated, from stage to stage, only in the presence of numerous external conditions and stimuli, beginning with the metabolic processes preceding the first cleavage of the egg-cell, until the final product of the development is as different from the first rudiment as adult animals and plants differ from their constituent cells.

To explain more clearly my conception of the nature of the process of development, especially in the relations that I conceive to exist between the rudiment and the adult, I shall conclude by reverting to my comparison between a human community and an organism.

As a man arises from an egg-cell by cell multiplication and cell differentiation, so the human community, a composite organism of a still higher nature, has arisen from separate human beings as its starting-point.

Culture and civilization are the wonderfully complicated results of the co-operation of many individuals united in society. By the manifolding of their relations and their combinations, men in society have brought about a higher complexity than man, left by himself, ever would have been able to develop from his own individual properties--a complexity that has arisen by the interaction of the same characters of many men in co-operation.

Similarly the activity of the egg in growth and cell-formation is an inexhaustible source of new complexity; for the self-multiplying systems of units, always binding themselves into higher complexes, continually enter into new interrelations, and afford the opportunity for new combinations of forces--in fact, of new characters.

Both cases--the course of the development of the egg-cell into a man, and of men into a state--depend upon epigenesis, not upon evolution.

The comparison may be carried into details.

The more complex and higher organisation of human society occurs in this fashion: of the numerous single individuals, all of which are endowed with the various incipient human characters, some individuals elaborate some incipient characters, others other characters, and these come to play correspondingly different parts. The special differentiation undergone by any individual depends upon the special place he comes to occupy in the whole of which he is a part, not upon really different organisation residing in him from his birth. Beside those characters which have developed specially in his case, there lie dormant the rudiments of all the characters possessed by men, and, under different conditions, these might have come to development.

Differentiation in multicellular organisms takes a similar course. Every cell, by doubling division of the egg, receives all the rudiments of its kind; of these rudiments, some in one set of cells, others in another, come to develop, according to the part of the whole in which the cells come to lie during the progress of the development, and according to the relations to the whole they come to assume. Thus, here they assume the characters of the external skin; there, they become gland-cells of the intestine; here, muscle-fibres; there, sense-cells or nerve-cells; in one place they serve the whole organism, in the form of blood-corpuscles, as agents for nutrition and respiration; there, becoming connective tissue or bone, they form skeletal elements of the body.

Thus, during the course of development, they are forces external to the cells that bid them assume the individual characters appropriate to their individual relations to the whole; the determining forces are not within the cells, as the doctrine of determinants supposes. The cells develop those characters that are suggested by their relation to the external world and their places in the whole organism.

But I must insist here that the subordination of the cells to the whole organism, in both multicellular animals and in plants, is much more complicated than that of the units to the human state. In the latter case, the individuals are separate from one another; they are independent organisms and are bound together only in social relations. None the less, consider how in a civilized state the apparently sovereign individual is conditioned in all his circumstances; how each change in the general state exercises an influence on the individual's disposition freedom of will, and method of life (dwelling, food, institutions, health); then reflect how much greater in the animal and the plant is the domination of the whole, and the subordination of the units, as in them cell is directly joined to cell--indeed, in most cases united materially by threads of protoplasm. In such cases the self-sufficiency of the cell as an elementary, living organism is so far prevented, that it becomes a subordinate part, with its function in dependence on the whole.

One other point our comparison will make clearer: I refer to the relation of the specific nature of the rudiment to the specific nature of the product of the rudiment.

The different organisations and qualities of the communities formed by different animals may be explained by the special characters of the animals forming them. Those of the bee colonies depend on the nature of bees; of ant colonies on the nature of ants; of the societies of men on the nature of men; indeed, in the latter case we see how they differ as they are formed by Italians, Germans, Slavs, Turks, Chinese, or Negroes. Similarly, the specific organisation of the cell determines the kind of animal which may be built up by it.

In my theory two assumptions of totally contrasting nature are made: I assume a germplasm of high and specific organisation, and I assume that this is transformed into the adult product by epigenetic agencies. To a certain extent, therefore, I reconcile the opposition between evolution and epigenesis, these opponents so prominent last century.

But my theory does not pretend to explain all the many problems involved in the course of organic development. In this respect it differs from Weismann's doctrine of determinants, as that is a closed system, finding within itself a formal explanation of all development. So far it seems to me an abandonment of explanation rather than an explanation; for it explains by signs and tokens that elude verification and experiment, and that cannot encounter concrete investigation. His explanation is no more than a description, in other words, of the visible events of development. To be more than this, it would be necessary to explain how in each case the biophores and determinants and ancestral plasms are constituted, and how they are arranged in the architecture of the germplasm so as to produce the development of the egg-cell in this or that fashion. It must, at the least, offer such possibilities as the structural formulae of chemists offer. But in the present stage of our knowledge Weismann's method is unpromising; it merely transfers to an invisible region the solution of a problem that we are trying to solve, at least partially, by investigation of visible characters; and in the invisible region it is impossible to apply the methods of science. So, by its very nature, it is barren to investigation, as there is no means by which investigation may put it to the proof. In this respect it is like its predecessor, the theory of preformation of the eighteenth century.

FOOTNOTES:

[17] The second section contains references to the following treatises:

C. V. NAEGELI: _Mechanisch-physiologische Theorie der Abstammungslehre_ (1884).

HERTWIG, OSCAR: _Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbelthiere_; 4th edit.

SACHS: _Lectures on Plant Physiology_; English edition, Clarendon Press.

VOECHTING: _Ueber die Theilbarkeit im Pflanzenreich und die Wirkung innerer und aeusserer Kraefte auf Organbildung an Pflanzentheilen._ _Pflueger's Archiv._, vol. xv., 1877.

Ibid.: _Ueber Organbildung im Pflanzenreich_, 1, 2; Bonn, 1878, 1884.

GOEBEL: _Beitraege zur Morphologie und Physiologie des Blattes._ _Bot. Zeit._, 1880.

PFLUeGER: _Die teleologische Mechanik der lebendigen Natur_; Bonn, 1877.

MAUPAS: _Sur le determinisme de la sexualite chez l'Hydatina senta._ _Comptes rendus des seances de l'Academie des Sciences_; Paris, 1891.

WEISMANN: _Die Allmacht der Naturzuechtung. Eine Erwiderung an Herbert Spencer_; Jena, 1893.

HERBERT SPENCER: _A Rejoinder to Professor Weismann._ _Contemporary Review_, 1893.

Ibid.: _Die Unzulaenglichkeit der 'Natuerlichen Zuchtwahl.'_ _Biol. Centralblatt_, vol. xiv., No. 6.

EMERY: _Die Entstehung und Ausbildung des Arbeiterstandes bei den Ameisen._ _Biol. Centralb._, vol. xiv., No. 2, 1894.

HAACKE; _Gestaltung and Vererbung_ (1894).

[18] The assumption of doubling division does not involve the assumption that the germinal mass is unalterable. Although I do not regard the process of division as a mechanism for breaking up the idioplasm into dissimilar groups of determinants, I regard the idioplasm--and here I agree with Naegeli--as only relatively stable. In course of time external and internal forces may slowly alter it. On the one hand, the idioplasm of the reproductive cells in the course of generations may slowly alter, while, on the other hand, the idioplasm of cell groups in an organism may acquire a local character in correspondence with their different topographical and functional positions in the whole creature, and in relation to their place in the organic division of labour, just as in human communities individuals become altered by the lifelong exercise of some calling.

Nor does the doctrine of doubling divisions conflict with those conclusions of pathology according to which, in the process of regeneration, cells and tissues give rise only to cells and tissues of their own order. For further details see my treatise, _Ei und Samen-Bildung bei Nematoden_, pp. 97-99. These slight suggestions are only to prevent misconceptions.

INDEX AND GLOSSARY

A

ACINETA, a group of protozoa, development of, 41.

Acquired characters, question of their inheritance, x.

Amphioxus, a marine animal, representative of the primitive vertebrate stock, experiments on eggs of, 61.

Anabolism, the formation of more complex chemical bodies by the agency of protoplasm, 86.

Animal cells, characteristic mode of growth, 111.

Antennularia, Loeb's experiment, 117.

Ants, polymorphism in, 125.

Ascidians, tunicate animals, 46.

Atavism, the occurrence in an organism of a character abnormal in it, but normal in an ancestor, 24.

B

Bees, polymorphism in, 125.

Beetroot, grafting experiments, 121.

Begonia, reproduction from leaves, 46.

BEET, experiments on rats, 73.

BERESOWSKY, skin-grafting, 75.

BEYERINCK, upon galls, 51.

Biophores. Each determinant, according to Weismann, is composed of a number of ultimate living pieces, the biophores, which are the active agents that direct the functions of a mature cell, ix, 22.

Blastosphere, an early stage in embryonic development; the embryo consists of a hollow sphere, the walls of which consist of a single layer of cells, and the cavity of which is called the segmentation cavity, xvii; explanation of formation, 97, 98.

Blood, transfusion of, 75.

BLUMENBACH, _nisus formativus_, 5; upon galls, 50.

Bone-grafting, 73, 74.

Bonellia, sexual dimorphism in, 122.

Bryozoa, a group of minute animals which form encrustations on seaweeds and stones, 46.

Buds, origin of, 28; reproduction and regeneration by, 46.

C

Cell, description of, 31; characters possible in, 88; differentiation of, in development, 112; as units in morphology and physiology, 113; Sachs on, 114; Voechting on, 114, 116.

Cell theory, relation of, to heredity, 31.

Centrosome, an organ of cells most obvious during nuclear division, 93.

Cerianthus, experimental heteromorphoses, 51.

CHABRY, destruction of segmentation sphere, 62.

Chromatin, a material found in the nucleus of cells, so called because it absorbs stains with avidity: germplasm and, viii, xiv; relation of, to specific character of cells, 36, 37.

Chromosomes, definite, visible bodies, as which the chromatin of a dividing nucleus appears, xiv, 93.

Crystal, growth of, compared with organic growth, 108.

Cione, experimental heteromorphoses, 52.

Clavellina, reproduction from buds, 46.

Cleavage-planes, the planes separating the daughter-nuclei, or daughter-cells, in the early division of a fertilised egg-cell, xvii; relation between appearance of, and structure of eggs, 95.

Coelenterata, a major division of multicellular animals, including such creatures as sea-anemones, corals, and jelly-fish, 46.

Continuity of the germplasm, 26.

Continuity of life, the doctrine opposed to spontaneous generation, 2.

Correlations, 118, 121.

D

DARWIN, pangenesis, 21.

Determinants. Each _id_ of germplasm is supposed by Weismann to be composed of minor pieces, arranged in a complicated fashion that is the result of the past history of the species. For every part of the body, large or small, that may be different in different individuals or species, there is, at least, one determinant in the _id_. The determinants are so grouped in the _id_ that they are liberated and become active when the time comes for the development of that part of the body they control, viii, 22; arguments against, 82; relation to cells, 87.

Determinates, the smallest parts of an organism which vary independently, and which are supposed by Weismann to be represented in the germplasm by special pieces, 23, 25.

Differentiating division, such a division of the nucleus as would result in daughter-nuclei unlike each other, and unlike the parent nucleus. The qualities of the parent nucleus are supposed to have been distributed between the daughter-nuclei, xi; absence of visible evidence for, xv, 25; objections to occurrence of, 34, 78.

Dimorphism, the appearance of the same species in two different forms, sexual dimorphism, 122, 124.

Disharmonic union in grafting, 70.

Double monsters, as examples of heteromorphosis, 63.

Doubling division. When an amoeba reproduces by simple division, the daughter-amoebae are identical, and each is identical with the parent except in size; from one amoeba two have been formed. A doubling division of the nucleus is such as would result in the formation of two nuclei alike in every respect, ix; visible evidence for, xv, 24; in unicellular organisms, 40; occurrence of, with differentiating division, 78.

DRIESCH, experiments on eggs, 54; separation of segmentation spheres, 60.

E

Echinoderms, a group of marine animals, of which the star-fish is the most familiar type, eggs of, 54.

Echinoidea, a group of echinoderms, 61.

Ectoderm, the tissue in an adult derived from the epiblast (which see), 19.

Egg, relation between structure and division of, 94; specific character of, 135.

EMERY, on polymorphism in ants, 128.

Endoderm, the tissue in an adult, derived from the hypoblast (which see), 19.

Enfoldment. See Evolution.

Epiblast. In the development of all multicellular animals, the young embryo soon becomes divided into two sets of cells, the epiblast and hypoblast; where a gastrula is formed, the outer layer of cells is the epiblast, the inner layer the hypoblast, xviii.

Epigenesis, the doctrine that the formation of a new individual is not the mere out-growing of particles hidden in the egg-cell, but the result of moulding external forces, xiii; Roux's definition of, 7; Weismann's denial of, 9; epigenetic explanation of stages in development, 98; summary of Hertwig's acceptance of, 136.

Evolution. Originally the term was applied, not to the origin of existing forms of life from common ancestors, but to the doctrine that every living creature contained within it the whole series of its future descendants, and that the growth of a living creature was evolving of one of these enfolded miniatures, xiii, 1, 2, 3; Roux's contrast of, with epigenesis, 6; the new evolution, 10; Hertwig's partial agreement with, 135, 136.

Experiment, Weismann's caution against, 10.

F

Fertilisation, the union of the nuclear matter of a male cell with the nuclear matter of a female cell, xii, xiv.

Foraminifera, a group of protozoa provided with shells, 44.

FOREL, on eyes of ants, 126.

Frogs' eggs, Hertwig's experiments upon; development of, under compression, 57-60.

Funaria, reproduction from chopped pieces, 46.

G

Galls, 50.

Gastrula, an early embryonic stage, most simply formed from the blastosphere by the invagination of one side of the wall, and consisting of a hollow sac, the walls of which are formed by two layers of cells, xviii, 60; formation of, 99.

Gemmules. See Pangenesis.

Germ, the youngest embryonic stage of an individual or organ, 10.

Germplasm, the substance supposed to be the material bearer of inherited qualities: Weismann's conception of, viii, 20; identification of, with nuclear matter, 21; account of Weissmann's theory, 21-28.

Germ-tracks, the hypothetical paths along which germplasm passes in an unaltered condition during development, 27; objections to, 81.

GOEBEL, on plasticity of plants, 120.

Grafting, 68, 70; of Hydra, 72; bone-grafting, 73, 74; skin-grafting, 74, 120, 121.

GRASSI, polymorphism due to food, 129.

Gregarines, a group of parasitic protozoa, development of, 41.

H

HAACKE, declaration that Hertwig is evolutionary, 135.

Haemoglobin, the red colouring matter of blood, 75.

Harmonic union in grafting, 70.

Heteromorphosis, explanation of, 49; cases of, 51, 52; embryonic cases, 54.

His, presence of foci in the germ, 13.

Histogenous, producing microscopical characters, 20.

Histology, study of the microscopical characters of cells and tissues, differentiation, 115.

Hydatina, determination of sex, 5; temperature, 123.

Hydra, regeneration in, 47; grafting of, 72.

Hydromedusae, a group of invertebrate animals, the typical members of which are branched colonies of polyps: Weismann's investigations on, viii, xii.

Hypoblast. See Epiblast, xvi.

Hypotrichous infusoria, a group of protozoa, 41.

I

_Ids_, hypothetical individual pieces, a number of which are supposed by Weismann to be present in the germplasm of every sexual cell, and each of which is supposed to contain the inherited material necessary for a complete new organism. It has been suggested that tiny beads seen within the chromosomes of a sexual cell are the _ids_, viii, 23, 33.

Idioblasts, Hertwig's name for hypothetical ultimate units of living matter, 22, 82; the ultimate units of living matter, according to De Vries, 22.

Idioplasm, as opposed to germplasm, which is the nuclear material of germ-cells; idioplasm is the nuclear material of tissue-cells, xi, 38.

Immortality, definition of, 82; of germ-cells, ix; of unicellular organisms, 17; of germ-cells, 80.

Individuality of cells, 115.

Invagination, the infolding of a layer of cells, as, for instance, in the transformation of a blastosphere into a gastrula, xvii.

Isotropism, explained in footnote, 33.

K

Karyokinesis, a complicated process of nuclear division, xiv.

Katabolism, the formation of less complex chemical bodies by the agency of protoplasm, 86.

L

Labile, unstable, constantly changing, 38.

LANDOIS, experiments on transfusion of blood, 75.

LEIBNITZ, on immortality, 82.

LOEB, on heteromorphoses, 49; on plasticity of animals, 117.

M

MAUPAS, experiments on sex of rotifers, 123.

Melons, determination of sex by temperature, 124.

Mesoblast, in the development of the coelomata, or three-layered multicellular animals; a third set of cells, the mesoblast, arises between the epiblast and hypoblast, xviii.

Monsters, relation of, to division of egg-cell, 63.

Mosaic theory of Roux, 56.

Morphoplasm, the general protoplasm of a cell, 35.

Multicellular organisms, those in which the body is composed of many cells, specialized in different directions; cell-division in, 43.

Mus, experiments on grafting among mice and rats, 74.

Myxomycetes, sometimes called 'slime fungi,' a group of low organisms, consisting of creeping masses of protoplasm with many nuclei, 33.

N

NAEGELI, biological units, 30; cross-fertilization and grafting compared, 69; heredity, 92; environment in development, 104; on plasticity of plants, 119; on specific characters of eggs, 134.

Nais, regeneration in, 47.

Notochord, formation of, from unusual cells, 117.

Nucleus, a specialized portion of the protoplasm of cells, different in chemical and physical properties (see Chromatin, Chromosomes), as the bearer of heredity, 19.

NUSSBAUM, views on origin of germ-cells, 17.

Nutrition, influence of, on development, 2.

O

OLLIER, bone-grafting, 73.

Ontogeny, the development of an individual from the egg upwards, 9.

Osteoblasts, cells which are the active agents in bone-formation, 73.

Ovogenesis, the formation of egg-cells in the ovary, 13.

P

Pangenesis, Darwin's provisional hypothesis, that the sexual cells were composed of minute particles (gemmules), given off by all the cells of the body, 21.

Periosteum, a cellular sheath of bones, 73.

Physiological units, Herbert Spencer's name for hypothetical ultimate units of living matter, 22.

Pistachio, influence of temperature on, 121.

Plant-cells, mode of growth, 110.

Plasomes, Hertwig's name for theoretical units of protoplasm, 32.

Plasticity of plant tissues, 117, 119, 120.

Pluteus, a free-swimming larval stage in the development of echinoderms, 54.

Podophrya, reproduction of, 41.

Polymorphism, the appearance of the same species in several different forms in ants and social insects, 125.

PONFICK, on transfusion of blood, 75.

Preformation, identical with the original meaning of evolution, which see.

Prothallus, the leaf-shaped green organism that grows from the spore of a fern and produces sexual organs, 49.

Pseudopodia, extensions of protoplasm beyond the general contour of the cell, 41.

R

Radiolaria, a group of protozoa, 44.

Regeneration in plants and animals, 45, 47.

Rhipsalis grafted on Opuntia, 71.

ROUX, contrast between epigenesis and evolution, 6; mosaic theory of, 56.

Rudiment, used here as a translation for the word _anlage_, which means the first plotting-out or beginning of a living structure. Darwin showed that rudimentary organs in adult creatures were for the most part vestiges of organs that had lost their use. In this treatise 'rudiment' is applied to an organ or structure in its incipient condition, whether that incipient state be visible in a young embryo, or a hypothetical structure in the germplasm, 6; latent rudiments, 37.

S

SACHS, on cells, 114; on reaction and protoplasm, 133.

Salix purpurea, reproduction from galls, 51.

SCHMITT, bone-grafting, 74.

Segmentation, the early division of a developing egg, xvii.

Segmentation spheres, the cells resulting from the early divisions of a developing egg, separation of, by Wilson and Driesch, 60.

Segmentation cavity. See Blastosphere.

Sex, determination of, by temperature, 123, 124.

Sexual cells (spermatozoa in male, ova or egg-cells in female), the nucleated pieces of protoplasm which are the starting-point of the new generation in sexual reproduction, origin of, 18.

Soma, the body of a plant or animal as contrasted with the reproductive cells contained within it, 45.

Somatic cells, the cells of the soma; mortality of, 17.

SPENCER, HERBERT, controversy with Weismann on polymorphism in insects, 125.

Spermatogenesis, the formation of spermatozoa in the testis, 13.

Spontaneous generation, 2.

Stolon, a strand of tissue connecting the individuals of colonial animals, 46.

STRASBURGER, the value of the nucleus in heredity, 13, 18.

T

Termites, polymorphism in, 125.

Transfusion of blood, 75.

Transplantation of bone, 73, 74.

TREMBLEY, grafting of Hydra, 72.

Triton, an amphibian, experiments on the egg by constriction, 64.

Tubularia, experimental heteromorphoses, 51.

Tunicata, a group of marine animals clad with a leathery tunic, 14.

U

Unicellular organisms, animals (protozoa) and plants (protophyta) with the simplest structure, each being a single cell: immortality of, 17; division doubling in, 40.

Unit, definition of a biological, 30.

V

Vegetative affinity, 66 _et seq._

Vertebrates, regeneration of lost parts, 47.

VOECHTING, experiments on grafting, 70; harmonic and disharmonic union, 70; on cells, 114, 116; on plasticity of plants, 117, 119; on grafting, 120.

W

WEISMANN and preformation, 8-10; caution against experiment, 12; sources of his theory, 20, 21; Hertwig's description of his theory, 22; absence of proof for differentiating division, 34; symmetry of egg and adult, 55; immortality of germ-cells, 17, 80, 82; germ-tracks, 83; doubling division, 102; controversy with Spencer, 125.

Willow, reproduction from slips, 46.

Wilson, separation of segmentation spheres of amphioxus egg, 60.

WOLFF, _Theoria Generationis_, 4.

Wounds, healing of, in relation to idioplasm, xii.

Y

Yolk, nutritive material stored in an egg-cell, xvi.

THE END.

BILLING AND SONS, PRINTERS, GUILDFORD.

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End of Project Gutenberg's The Biological Problem of To-day, by Oscar Hertwig