Part 45

The whole of the reconstruction of the external form of the fly takes place in a similar manner. Not only the limb, the head, the stigmata, but the skin itself is formed anew from imaginal disks. In each of the abdominal segments three pairs of little cell-islands are formed during larval life, and these only enter on the stage of formative activity after pupation, when they multiply rapidly and grow together to form a segment, whose size, form, and external nature is determined by them. But it is well known that the abdominal segments of the fly differ from those of the larva very markedly and in every respect, so that each cell-island must contain determinants which are quite different from those in the skin-cells of the corresponding larval segments. These last break up at the beginning of pupahood, while the former begin to grow vigorously, and to spread themselves out. The most remarkable fact about the whole business, and it seems to me also the most instructive, is that these imaginal disks frequently appear for the first time during larval life, as I found in the case of a midge, _Coretha plumicornis_, in regard to the disks of the thorax, and as Bruno Wahl[23] has recently demonstrated in the case of the abdominal cell-islands. Since in the young larva the position of the subsequent imaginal disks is occupied by cells which apparently in no way differ from the rest of the skin-cells, and are also exposed to precisely the same external and internal influences, the origination of the imaginal cells from these can only depend on differential cell-division; the primordial cell of each imaginal disk must have separated at the beginning of disk-formation into a larval and an imaginal skin-cell.

[23] Bruno Wahl, _Ueber die Entwickelung der hypodermalen Imaginalscheiben im Thorax und Abdomen der Larve von 'Eristalis' L., Zeitschr. f. wiss. Zool._, Bd. lxx. 1901.

In insects in which the larva and the imago differ widely, the perfect insect, as regards all its principal parts, is already represented in the larva, namely, in particular cells which lie among those of the corresponding larval parts, and do not visibly differ from these, although they are equipped with quite different determinants, and consequently enter on their formative activity much later, and give rise to quite different structures. As the determinants of the whole animal with all its parts are contained in the ovum, so those of the parts of its imaginal phase are contained in these cells of the imaginal disks.

In addition to all this, we have incontrovertible evidence in favour of the theory of determinants in the independent phyletic variations of the individual stages of development, on which depends the whole phenomenon of 'metamorphosis' which we have just been considering. How could the larval stage have become so different from the imago-stage, if the one were not alterable by variation arising in the germ without the other being affected? If this absolute independence of the transmissible variability of the individual stages were not an indispensable assumption in the explanation of metamorphosis and other phenomena of development, I should regard an attempt at a theory of development without determinants as justifiable. But I am forced to see in this fact alone an invalidation of all epigenetic theories of development, that is, of all theories which assume a germ-substance without primary constituents, which can produce the complicated body solely by varying step by step under the influence of external influences, both extra- and intra-somatic. It is possible to conceive of an ovum in which the living substance is of such a kind that it must vary in a definite manner under the influence of warmth, air, pressure, and so on, that it must divide into similar, and subsequently also into dissimilar parts, which then interact upon each other in diverse ways and give rise to further variations, which in their turn result in differentiations and variations, till ultimately we have the whole complicated organic machine complete and 'finished' in every detail. Certainly no mortal could make any pronouncement as to the constitution of such a substance, but even if we assume it, for the nonce, as possible, how can we account for the transmissible variation of the individual parts and developmental stages, on which the whole phylogenetic evolution depends?

As the development of the butterfly exhibits the three main stages of caterpillar, pupa, and perfect insect, each of which is independently and hereditarily variable, and therefore implies a something in the germ, whose variation brings about a change in the one stage only, so the ontogeny of every higher animal is made up of numerous stages, which are all capable of independent and transmissible variation. How else should we human beings, in our embryonic phase, still possess the gill-arches of our fish-like ancestors, although much modified and without the gills? Truly, he who would seek to deny that the stages of individual development are capable of independent and transmissible variation must know very little about embryology. But if the facts are as stated, how can they be reconciled with the conception of a germinal substance developing in epigenetic fashion? Every variation in this substance would affect not only the whole _succession of stages, but the whole organism with all its parts_. In this way too, then, we are driven to the conclusion that there must be something in the germ whose variation causes variation only in a particular part of a particular stage. This something we define in our conception of the 'primary constituents' (_Anlagen_)--the determinants. These are not to be thought of either as 'miniature models,' or even as the 'seeds' of the parts; they alone cannot produce the part which they determine, but they effect changes in the cell in which they become active, causing it to vary in such a manner that the formation of the relevant part results. While I conceive of development as a continuous process, I supplement this with the idea that from within, namely, from the nuclear substance, new, directive, 'determining' influences are continually being exerted on the developing cells.

I can hardly think of a better proof of the necessity of this assumption than that furnished by Delage, one of the most acute biologists of France, who, in his comprehensive book on _Heredity_, has striven to replace the theory of determinants by something simpler. Delage rejects all 'primary constituents' (_Anlagen_) in the germ, all 'particules représentatives,' as much too complicated an assumption, and thinks it possible to work with the conception of a germ-plasm which is about as simple as the cell-substance of a Rhizopod, that is to say, a protoplasm of definite chemico-physical constitution and composition. Leaving out of account the consideration that the protoplasm of an amœba is scarcely of such extreme simplicity, but is certainly made up of numerous differentiated and definitely arranged biophors, how could such an extremely simple ('éminemment simple') constitution of the ovum as is here assumed give rise to such a complicated organism, the individual parts of which are capable of independent and transmissible variation? According to Delage it does so because the ovum, though not containing 'all the factors requisite for its ultimate resultant,' does contain 'un certain nombre des facteurs nécessaires à la détermination de chaque partie et de chaque caractère de l'organisme futur'! Determinants after all, it may be said, but that is far from the truth! It is not primary constituents that the germ contains, according to Delage, it is chemical substances, for instance muscle substances, probably 'les substances caractéristiques des principales catégories de cellules, c'est-à-dire, celles qui, dans ces cellules, sont la condition principale de leur fonctionnement.' All these must be contained in the ovum. How they are to reach their proper place in the organism, how the 'characteristic chemical substance' of a mole is to land just behind the right or left ear of the fully formed man, is not stated. But apart from this, there is a much deeper error in this assumption of specific chemical substances in the ovum as an explanation of the phenomena of local hereditary variation, and I have already touched upon it: chemical substances are not vital units, which feed and reproduce, which assimilate and which bear a charm against the assimilating power of the surrounding protoplasm. They would necessarily be modified and displaced in the course of ontogeny, and would therefore--no matter where they had been placed at first--be incapable of performing all that Delage ascribes to them. Either the germ contains 'living' primary constituents, or it is, as Delage maintains, determined chemico-physically; but in the latter case there is no scope for hereditary local variation. Delage must either renounce the attempt to explain this, or he must transform his 'substances chimiques' into real and actually living determinants.

Thus from all sides we are forced to the conclusion that the germ-substance on the whole owes its marvellous power of development not only to its chemico-physical constitution, whether that be eminently simple or marvellously complex, but to the fact that it consists of many and different kinds of 'primary constituents' (_Anlagen_), that is, of groups of vital units equipped with the forces of life, and capable of interposing actively and in a specific manner, but also capable of remaining latent in a passive state, until they are affected by a liberating stimulus, and on this account able to interpose successively in development. The germ-cell cannot be merely a simple organism, it must be a fabric made up of many different organisms or units, a microcosm.

Yet another train of thought leads us to the same idea, and this has its roots in the extraordinary complexity of the machine which we call the organism.

The botanist Reinke has recently called attention once again to the fact that machines cannot be directly made up of primary physico-chemical forces or energies, but that, as Lotze said, forces of a superior order are indispensable, which so dispose the fundamental chemico-physical forces that they must act in the way aimed at by the purpose of the machine. To produce a watch it is not enough to bring together brass, steel, gold, and stones; to produce a piano it is not enough to lay wood, iron, leather, ivory, steel, &c., side by side, but these stuffs must be brought together in a definite form and combination. In the same way, the mere juxtaposition of carbon and water does not result in a carbohydrate like sugar or illuminating gas; the component elements only yield what is desired when they are placed in a particular and absolutely definite relation to each other, in which they so act upon and with one another that sugar or illuminating gas results, and the same is true of the component elements of a watch or of a piano. In the watch and in the piano this relation is arranged by human intelligence, by the workmen who form the different materials and put them together in the proper manner. In this case, then, human intelligence is, as Reinke says, the 'superior force' which compels the energies to work together in a particular way.

But organisms also are machines which perform a particular and purposeful kind of work, and they are only capable of doing so because the energies which perform the work are forced into definite paths by superior forces; these superior forces are thus 'the steersmen of the energies.' There is undoubtedly a kernel of truth in this view, and I shall return to it. Reinke, however, uses it in a way which I cannot follow; that is, he infers from it a 'cosmic intelligence' which puts these superior forces into the organisms, and thus controls these machines to purposeful work, as the watchmaker puts 'superior forces' into the watch by means of wheels, cylinders, and levers. In one case it is human intelligence which controls the 'superior forces,' in the other 'cosmic' intelligence. I cannot regard this reasoning from analogy as convincing, because, in the first place, these 'superior forces' are not 'forces' at all. They are constellations of energy, co-ordinations of matter and the energies immanent therein under complex and precisely defined conditions, and it is a matter of indifference whether chance or human intelligence has brought them together. If we take Reinke's own example of carbohydrates it is certain that our coal-gas is due to the intelligence of man, which brings together the carbon and the water in such a way that coal-gas must arise. The 'superior forces' must here be looked for in the arrangements of the coke-stove, and, in the second place, in the intelligence of man. But when decaying plants in the marsh form another carbon-compound, marsh-gas, where do the directing 'superior forces' come in? Surely only in the fortuitous concomitance of the necessary materials and the necessary conditions. Or may 'cosmic' intelligence have established this laboratory in the marsh? If not, what can compel us to refer the formation of dextrin or starch in the cells of the green leaves of plants to 'superior forces' which are placed in them by 'cosmic' intelligence? I am far from believing that the great and deep problem here touched upon can be put aside in any off-hand manner, but I feel sure that it will never be solved by word-play about energies and 'superior forces.'

Let us return to the kernel of truth in Reinke's thesis; it lies in this, that, while the working of a machine does really depend on the forces or energies which are bound up with the stuffs of which it consists, it also depends on a particular combination of these stuffs and forces, on a particular 'constellation' of them, as Fechner expressed it. In the watch these 'constellations' are the springs, the wheels, &c., and their position in relation to each other; but in the organism they are the organs, down to the cells and cell-parts; for the cell too is a machine, indeed a very complex one, as its functions prove. There are thousands of kinds of 'constellations' of elementary substances and forces which condition the activity of the living machine, and only when all these constellations are present in the proper manner and in the proper relations to each other can the functions of the organism be properly discharged.

But the living machine differs essentially from other machines in the fact that it constructs itself; it arises by development from a cell, by going through numerous 'stages of development.' But none of these stages is a dead thing, each is itself a living organism whose chief function is to give rise to the next stage. Thus each stage of the development may be compared to a machine whose function consists in producing a similar but more complex machine. Each stage is thus composed, just like the complete organism, of a number of such 'constellations' of elementary substances and elementary forces, whose number in the beginning is relatively small, but increases rapidly with each new stage.

But whence come these 'constellations' or, to keep to our metaphor, the levers, wheels, and cranks of each successive stage in the making of the organic machine? The epigenetic theory of a germ-plasm without primary constituents answers by pointing to internal and external influences which cause the germ-plasm, originally homogeneous, to differentiate gradually more and more, bringing it into the most diverse 'constellations.' But how can such influences introduce new springs, levers, and wheels of a quite specific kind, as must be the case if apparently similar germinal substances are to give rise to two such different animals as a domestic duck and a teal? The cause must lie in the invisible differences in the protoplasm, opponents will answer, and we with them. But our studies up to this point have shown us that the differences cannot be merely elementary differences, cannot be merely of a physico-chemical nature depending on the composition of the raw material and the implicated energies; they must depend on the definite co-ordination of substances and energies, in other words, on the occurrence of 'constellations' of these. Thus the germ-plasm must be composed of definite and very diverse combinations of living units, which are themselves bound up in a higher 'constellation,' so that they act as a living machine at the first stage of development, and liberate into activity the already existing constellations of the second stage. The second stage in the series of living machines which arise successively from each other liberates the sleeping 'constellations' for the third, and so on.

These 'constellations' of matter and energy are the biophors, the determinants, and the 'groups of determinants' which we may think of as disposed in a manifold overlapping series. That they do not enter into activity all at once, but successively take their part in development, seems to me a necessary consequence of their successive origin in the phylogeny; and the ontogeny, as we shall see later, arises through a modified condensation of the phylogeny. Now since every new determinant that arises in the course of phylogeny can only develop by division and subsequent variation from the determinants which were previously active at the same place in the organism, it is quite intelligible that later on, when the phylogeny has been condensed in the ontogeny, they should not enter upon their active stage at the same time as their phyletic predecessors, but after them. The theory of Oscar Hertwig, who starts from a germ-plasm without primary constituents, that all parts of the germ-plasm become active at the same time, seems to me quite untenable. How could the wheels, levers, and springs of the complete vital machine, which arose so very slowly in the course of phylogeny, arise to-day in the ontogeny in such rapid succession unless they were already present in the germ-plasm and only required to be incited to activity, that is, liberated by the stage preceding them? Even Fechner supported this view when he supposed that the interaction and mutual influences of the parts in the organism, that is, of the 'constellations,' gave rise of themselves to the succeeding stage, that is to say, to the new constellations peculiar to the succeeding stage. To this Reinke reasonably objected that it was like expecting the window frames of a house in process of building to produce the panes of glass. The panes in the organism only develop in the window frames if their determinants have been present in the germ-plasm from the beginning, and are liberated by the development of the frames, just as the activity of the glazier is liberated by the sight of the completed frames. Neither new panes nor new determinants could be produced rapidly; the former must be manufactured in the glass factory, the latter in the developmental workshop of the form of life in question, which workshop we call its phylogeny. But just as it is unnecessary to erect a new glass factory for each new house that is built, so the development of each individual does not require the establishment each time of those numberless life-factories--the constellations--whose business it is to produce anew the wheels, levers, springs, and cylinders of the developmental machinery at each stage, for they are all provided for in the germ-plasm, and it is only on this account that they are capable of hereditary variation.

I have already directed attention to some embryological facts which seem to be contradictory, if not to the germ-plasm theory itself, at least to the assumption it makes that the germ-plasm is analysed out during the ontogeny; and something more must be said on this head. I refer to the numerous facts brought to light through the science of developmental mechanics founded by Wilhelm Roux, and particularly to the investigations as to the prospective significance of the segmentation-cells of the animal ovum.

Among these investigations we find experiments in compressing certain eggs (sea-urchin's) in the early stages of segmentation. The blastomeres are prevented by artificial pressure from grouping themselves in the normal manner; they are compelled to spread out side by side in the _same plane_. If the pressure is removed, they change their grouping, and yield a normal embryo. I will not here discuss whether these results can only be interpreted as showing that each segmentation-cell has the same prospective significance, and that it is only its relative position which decides what part of the embryo is to be formed from it; this could not be done without going into great detail; I therefore assume it to be true, and confine my survey to the second group of experiments, those on isolated segmentation-cells.

It has been shown that in the eggs of the most diverse animals, for instance in the sea-urchin once more, each of the two first blastomeres, if separated from one another, can develop into a complete larva. Indeed, in the eggs of sea-urchin and some other animals each of the first four, or any of the first eight, blastomeres, and indeed any segmentation-cell during the earlier stages, possesses the power of developing to a certain point, namely, as far as the so-called 'blastula-larva.' This seems to contradict a theory which assumes that the primary constituents become separated in the successive stages of ontogeny. But in the first place the blastomeres of all animals do not behave in this way, and, moreover, the facts can be quite well explained without entirely renouncing the assumption of the segregation of the determinant-complexes. It is only necessary to assume that the segmentation-cells, which develop in the isolated condition as if they were intact eggs, still contain the complete germ-plasm, and that the differential segregation into groups of determinants with dissimilar hereditary tendencies takes place later. This would certainly load the theory with further complications, and I shall not enter into the question here, since the facts which we should have to consider are as yet by no means undisputed.

But in any case the facts of developmental mechanics referred to, which we owe to numerous excellent observers of the last decade,--I need only name W. Roux, O. Hertwig, Chun, Driesch, Barfurth, Morgan, Conklin, Wilson, Crampton, and Fischel--not only leave the essential part of the germ-plasm theory untouched, but rather strengthen than endanger its more subordinate points, such as the assumption of a segregation of the components of the germ-plasm in the course of ontogenesis.

As to the fundamental ideas expressed in the theory, I have already shown that these remain unaltered, even if we do not assume a disintegration or segregation of the germ-plasm, but think of all the developing cells as equipped with the complete germ-plasm. In that case the determinants would be liberated to activity solely by specific stimuli. But in regard to the assumption of disintegration, it must be noted that the facts cited relative to the sea-urchin's ova do not by any means hold true of the eggs of all animals.