Part 39

All the more recent experiments on 'merogony,' that is, on the development of fragments of the ovum, confirm this view. Thus Boveri had already observed that even small pieces of sea-urchin ova which did not contain the nucleus of the ovum developed, after the spermatozoon had entered them, into small but otherwise normal larvæ of the species. More recently Hans Winkler proved the same thing for the ova of plants, by dividing the ovum of a marine alga (_Cystosira_) into two pieces, then fertilizing these with water containing sperms, with the result that he got from both pieces, the nucleated and the non-nucleated, an embryo of normal appearance. In the latter it could only have been a 'paternal' nucleus which directed the development.

To sum up. Our investigation into the meaning of amphimixis has led us to the conclusion that it consists in the union of two equal complements of hereditary substance, contributed by two different individuals, into one unified nucleus, and that the sole immediate result of this is _the combination of the hereditary tendencies of two individuals in one_. Among multicellular organisms this one individual of dual origin always implies the beginning of a new life, since amphimixis is indissolubly associated with reproduction, and even among unicellular organisms it can hardly be disputed that the two Infusorians which separate after conjugation are no longer the same as they were before. After amphimixis they must contain a different combination of hereditary substance from what they had before, and this must reproduce the parts of the animal in a somewhat modified form. This is theoretically beyond doubt, although it can scarcely be established by observation.

We thus know now what 'fertilization' is. Through the labours of the last decade the veil has been torn from a mystery of nature which for thousands of years confronted humanity as unapproachable; a riddle has been solved for the solution of which a few centuries ago men did not even dare to hope. Not a few have taken part in these labours; some I have already named, but it is impossible that I should here mention all who have shared in the achievement by observation or reflection. Whoever has helped it on even a single step may say to himself that he has taken an active part in bringing about what must be called essential progress in human knowledge.

But in the science of nature every new solution implies the cropping up of a new riddle, and we are immediately confronted with the problem, Why should nature, in the course of evolution, have interpolated this process of the mingling of different hereditary substances almost everywhere in the organic world? This, however, is a problem which we cannot attack until we have first made ourselves more fully acquainted with the phenomena of inheritance, and have attempted to reason back from these to the nature of the hereditary substance. We must, in short, think out a theory of heredity.

LECTURE XVII

THE GERM-PLASM THEORY

Conception of the 'id' deduced from the process of fertilization--Hereditary substance, 'idioplasm' and 'germ-plasm'--'Idants'--Evolution or Epigenesis--Herbert Spencer's uniform germinal substance--Determinants--Illustrations: _Lycæna agestis_--The leaf-butterflies--Insect metamorphosis, limbs of segmented animals--Heterotopia--The ultimate living units or biophors--Number of determinants--Stridulating organ of the grasshopper.

IN proceeding to expound the theory of heredity which has shaped itself in my mind in the course of my own scientific development, I should like to begin by pointing out that the hereditary substance of the germ-cell of an animal or of a plant contains not only the primary constituents (_Anlagen_) of a single individual of the species, but rather those of several, often even of many individuals. That this is so can be proved in several ways.

I start from what I hold to be the proved proposition, that the chromatin substance of the nucleus is the hereditary substance. We have seen that this is present in the germ-cells of every species in the form of a definite number of chromosomes, and that in germ-cells destined for fertilization, that is, in sex-cells, this number is first reduced to half, the reduction being effected, as is now proved in regard to a whole series of animals, by the two last cell-divisions, the so-called maturation divisions.

We know that the full number is only reached again through amphimixis, by which process the half number of chromosomes in the male and female germ-cells are united in a single cell, the 'fertilized ovum,' and in a single nucleus, the so-called segmentation nucleus. Thus the hereditary substance of the child is formed half from the paternal, half from the maternal hereditary substance, and we have seen that this remains so during the whole development of the child, since, at every succeeding cell-division each of the paternal and each of the maternal chromosomes doubles by dividing, and the resulting halves are distributed between the two daughter-nuclei.

Now if the complete hereditary substance of a germ-cell before the reducing divisions contains potentially all the primary constituents of the body, which it does as a matter of course, then it follows that after the reduction each germ-cell must either contain only half the primary constituents of the parents or all the primary constituents must be contained in the half number of chromosomes. The latter seems to me the only possible assumption, as I shall immediately proceed to show, and this is as much as to say that the primary constituents of at least two complete individuals must be contained in the chromosomes of the segmentation nucleus.

That this conclusion is correct is obvious from the fact that a whole, that is, a perfect individual with all its parts, develops from the ovum, and not a defective one. For suppose that each mature germ-cell contained only half the primary constituents of the body, it would be impossible that these halves should always exactly complete themselves to form a whole embryo when they are brought together in fertilization, after having been halved by mere chance during the preceding reducing division; it would be much more likely to happen that they did not complete themselves, and that their union would therefore result in an individual with certain parts wanting. If, for instance, in the sperm-cell only the anterior half of the body was potentially present, and this united with an ovum which likewise contained only the primary constituents of the anterior half, the embryo resulting from their union would lack the posterior half of the body, and so on. Of course so rough a division of the primary constituents is not to be thought of, but however fine we can imagine the halving of the mass of primary constituents to be, there would never be any guarantee that the two cells uniting in amphimixis would complete the mass of primary constituents again; indeed, the chance that the two exactly complementary halves of the mass would meet would rather become less the finer and more complex one imagines the halving by reducing divisions to be. A perfect embryo with all its parts would rarely arise, but now one group of parts, now another would be wanting, while another group might be developed double, or at least would be doubly present in the primary constituents.

But in addition to this the facts of inheritance show us that the resemblance to mother and father may express itself simultaneously in all the parts, or at least in the same parts of the child, as may be seen with especial clearness among plant-hybrids, and thus the conclusion is inevitable that even in the half number of chromosomes all the primary constituents of the whole body are present.

Let us go a generation further. If the species possess four chromosomes the child will have in its cells two maternal chromosomes (_A_) and two paternal chromosomes (_B_); what form will this proportion take in the germ-cells produced by the child? The maturation division can effect the reduction to two chromosomes in different ways; there may, for instance, be two paternal chromosomes (_B_) left in the one, and two maternal chromosomes (_A_) in the other daughter-cell, or one paternal (_B_) and one maternal (_A_) in the one, and a similar combination in the other cell. Let us follow the latter case further. A sperm-cell which contained the combination _A_ and _B_ might meet in amphimixis with an egg-cell of different origin also containing a similar combination of chromosomes, let us say a chromosome _C_ from the mother, and a chromosome _D_ from the father. We should then have in the segmentation nucleus of the fertilized ovum four different chromosomes, each of which contained the hereditary substance of one grandparent; we should have the four chromosomes, _A_, _B_, _C_, _D_, as the hereditary substance of the grandchild.

_But since, as we have seen, the halved hereditary substance still contains the whole mass of primary constituents, each one of these chromosomes must contain the collective primary constituents of the whole body of the relevant grandparent_[18]. _The hereditary substance in the fertilized ovum thus consists of several complexes of primary constituents (chromosomes) each of which (an 'id') comprises within itself all the primary constituents of a complete individual._

[18] When I say the 'collective' primary constituents of the whole body of the grandparent this is not expressing it quite precisely, for, as we shall see later, each individual must arise from the co-operation of different chromosomes of different origin, not merely from one of the chromosomes contained in its germ-plasm. In the example given above, the body of each grandparent cannot have arisen only from a single chromosome, which was transmitted to his grandchild, but from the co-operation of this chromosome with three others, which have distributed themselves along other genealogical paths. But this does not affect the above chain of reasoning, for here it is not a question of whether all the primary constituents of the grandparent are present in the child--that can never be the case--but whether the primary constituents transmitted by him represent the whole body of an individual.

It can be made clear in yet another way that, as a consequence of sexual reproduction, the germ-plasm of each species must be composed of several 'ids,' _individually different_. Let us assume that there was as yet no amphimixis, and that we could look on at its introduction into the organic world; the hereditary substance of the beings which had previously lived and multiplied by division would consist of more or less numerous chromosomes similar to each other, so that, for instance, each individual would contain sixteen identical 'ids.' But if amphimixis were now to take place for the first time, in the same manner as it does to-day--that is, after the reduction of the number of the ids to half--in the first amphimixis eight paternal ids would unite with eight maternal ids to form the germ-plasm of the new individual, as is indicated in Fig. 87 by a circle of spheres, of which ten are white and ten black as a sign of their difference. We may think of the figure as representing the 'equatorial plate' of a nuclear spindle with its ids arranged in a circle. Now, if two organisms of this generation, with two kinds of ids, unite in amphimixis after previous reduction of the ids, we have figure _B_, in which the paternal ids (_pJ_) are seen to the left of the line and the maternal ids (_mJ_) to the right, while each semicircle is in its turn made up of two kinds of ids, those of the grandparents (_p_^2_J_ and _m_^2_J_, _p_^2_J_^1 and _m_^2_J_^1). The figures _C_ and _D_ show the two following generations, in which the number of identical ids is each time reduced to half, because eight strange ids are again mingled with them; in _C_ only two ids are still identical, and in _D_ all the ids are individually different, because they have come from different ancestors of the same species. Of course this would only be the case if inbreeding were excluded, because through it the ids of the same forefathers from two or more sides would meet; but prolonged inbreeding is a rare exception in free nature.

I shall now call the hereditary substance of a cell its 'idioplasm,' after Nägeli's example, although he sought it in the cell-substance, not in the nucleus, and had a different theoretical conception of its mode of action. It was he, however, who conceived and established the idea of the idioplasm as the bearer of the primary constituents, an _Anlagensubstanz_, determining the whole structure of the organism in contrast to the general nutritive protoplasm. Every cell contains idioplasm, since every cell-nucleus contains chromatin, but I call the idioplasm of the germ-cells _germ-plasm_, or the primary-constituent-substance of the whole organism, and the complexes of primary constituents necessary to the production of a complete individual--whose presence we have just shown to be theoretically necessary--I call _ids_. In many cases these 'ids' might be synonymous with chromosomes, at least in all the cases in which the chromosomes are simple, that is, are not composed of several similarly formed structures. Thus in the salt-water Crustacean, _Artemia salina_, which possesses 168 minute granular chromosomes, each of these chromosomes must be regarded as an id, for each can in certain circumstances be thrown out from the ovum by the reducing division, or it can be brought into the most various combinations with other chromosomes by fertilization. Each of them must therefore consist of perfect germ-plasm in the sense that all the parts of an individual are virtually contained in it; _each is a biological unity, an id_. But when we see in many animals larger band-shaped or rod-shaped 'chromosomes,' and when these are composed of a series of granules, as they are, for instance, in the often mentioned _Ascaris megalocephala_, each of these granules is to be regarded as an id. In point of fact, we find, instead of the two or four large rod-shaped chromosomes of _Ascaris megalocephala_, a larger number of smaller spherical chromosomes in other species of _Ascaris_.

Compound chromosomes consisting of several ids, such as all rod or band-like elements of the nuclear substance probably are, I designate 'idants.' That they are composed of several individual ids is not always clearly apparent because of the smallness of the object, and even in larger ones this may only be seen in certain stages. Thus we have in Fig. 88, _A_ and _B_, two 'mother-sperm-cells' of the salamander; _A_ at an earlier stage, in which the individual ids are not visible; _B_ at a later stage, in which the band has split, and the rosary-like structure has become at once apparent. It is not possible, then, to see at once whether each chromosome corresponds to one or to several ids. A more exact investigation of the processes of reducing division has shown that there are chromosomes of simple spherical form, that is, composed of several ids whose 'plurivalence' cannot be directly recognized, but can only be inferred from their further development; there are bivalent chromosomes of double value and quadrivalent chromosomes of fourfold value, which we have to think of as made up of two or four ids. It would lead us too far to go into this more precisely, nor does it fall within the scope and intention of these lectures to inquire into these intimate and still disputed details.

The germ-plasm of every species of plant or animal is thus composed of a larger or smaller number of ids or primary constituents of an individual, and it is through the co-operation of these that the individual which develops from the ovum is determined.

We have further to inquire what conception we can form of the constitution of an id and of its mode of operation. I have already spoken of 'primary constituents' (_Anlagen_) of which the germ-plasm consists, but what right have we to think of the parts of an animal as already contained in the germ in any form whatever? Is it not equally possible that the germ consists of parts, none of which bear any definite relation in advance to the parts of the finished animal? Might not the germ-cell, along with its nucleus, undergo transformations and regular changes which would successively give rise to new conditions, namely, the different stages of development, until finally the complete animal was attained?

We stand here before an old problem, before the two opposed interpretations--the theory of 'Evolution' and the theory of 'Epigenesis,' which were first ranged against each other long ago, and which are a cause of strife even now, although in somewhat different guise.

The theory of 'Evolution' is especially associated with the name of Bonnet, who elaborated it in detail in the eighteenth century. It maintains that the development of the ovum to the perfect animal is not really a new creation, but only an unfolding of invisible small parts, which were already present in the ovum. It assumes that the parts of the perfect organism are already preformed in the ovum, and on this account it is called the 'Preformation Theory.' Bonnet often speaks of the preformation of the perfect animal in the germ as a 'miniature model,' although his conception of 'evolution' was not really so crude as has been often alleged. He expressly emphasized that this miniature model was not exactly like the perfect animal, but consisted of 'elementary parts' only, which he thought of as a net whose meshes were filled up during development and by means of nutrition with an infinite number of other parts. But after all, his conceptions, and those of his time generally, were very far removed from the biological thinking of our own day, as may perhaps be most readily understood when I mention that he regarded death and decay as an 'involution,' as a folding back, so to speak, by means of which all the parts gained though nutrition were removed again, so that the net of the miniature model shrank together to the invisible minuteness that it had in the ovum. So it remained, he fancied, till it was reawakened at the resurrection, using the term in the religious sense! He afterwards dropped this fancy, because the objection was made to it that human beings who had lost a leg or an arm in this life would necessarily be maimed at the resurrection!

In Bonnet's time the facts of development were quite unknown, and not even the stages of the development of the chick from the egg had been observed. When this was afterwards done the prevalent theory of 'evolution' necessarily collapsed, for men saw with their own eyes that a miniature model of the chick did not gradually grow into visibility and ultimately into the young chick, but that first of all parts showed themselves in the egg which bore no resemblance at all to the chick, that these first rudiments were then altered, and that through continual new formations and transformations the chick finally appeared. Upon this K. von Wolff based his theory of 'Epigenesis,' or development through new formations and transformations. He maintained that the doctrine of 'Evolutio' was false; that there is no miniature model invisibly contained within the egg; but that from the simple egg-substance there arises, through the agency of the formative powers inherent in it, a long series of stages of development, of which each succeeding one is more complex than the one before, until ultimately the perfect animal is reached.

This certainly marked considerable progress, for it meant the beginning of a science of embryology, that is, the science of the form-development of the animal or plant from the ovum. The result was not so important in its theoretical aspect, for though the knowledge had been gained that the young animal goes through a long series of different stages, it had not been discovered how nature works this wonder and causes an animal of complex structure to arise from the apparently simple substance of the ovum. A solution of the difficulty was found by attributing to the ovum a formative power, afterwards called by Blumenbach the _nisus formativus_, which possessed the capacity of developing a complex animal from the simple 'slime,' or, as we should say, the simple protoplasm.

If we contrast the strictly theoretical part of the two theories, we find that Bonnet regarded the ovum as something only apparently simple, but in reality almost as complex as the animal which developed from it, and that he thought of the latter, not as being formed anew, but as being unfolded or evolved. That is to say, he thought that rudiments present from the outset in the ovum gradually revealed themselves and became visible. Wolff, on the other hand, regarded the ovum as being what it seemed, something quite simple, out of which only the _nisus formativus_ could, by a series of transformations and new formations, build up a new organism of the relevant species.

Wolff's Epigenesis routed Bonnet's theory so completely from the field that, until quite recently, epigenesis was regarded as the only scientifically justifiable theory, and a return to the 'evolutionist' position would have been looked upon as a retrograde step, as a reversion to a period of fancy which had been happily passed. I myself have been repeatedly told, with regard to my own 'evolutionistic' theory, that the correctness of epigenesis was indisputably established, that is, was a fact, verifiable at any time by actual observation!

But what are the facts? Surely only that there is a succession of numerous developmental stages, which we know very precisely in the case of a great many animals, and that the miniature model which Bonnet assumed to be in the egg does not exist. Both these facts are now no longer called in question. But that does not furnish us with a theory of development, for theory is not the observation of phenomena or of a series of phenomena, _it is the interpretation of them_. Epigenesis, as formulated first by Aristotle and again by Harvey, Wolff, and Blumenbach, certainly offered an interpretation of development, not, however, by referring only to what was observable, but by going far beyond it; on the one hand taking the _appearance_ of a homogeneous germ-substance for reality, and, on the other, assuming a special power, which caused a heterogeneous organism to arise from a homogeneous germ.