Now, is there any part of these processes that has to do with the breaking of the nuclear contents into groups of determinants with different qualities? By no means. The egg divides into many pieces, because such division is a general property of cells, and it is not associated with separate, special material bearers. The appearance of spaces between the cells, resulting from division, is due to forces some of which reside within the single cells, some of which come from without. In especial, the assumption of a spherical shape--an assumption occurring also to a greater or less degree when the results of division leave each other--is caused by the yolk actively arranging itself round the two nuclei as centres of attraction. The attempt to become spherical is opposed by other forces, in accordance with which the cells resulting from division press against each other. These forces that press the cells together seem to increase, as the size of the cells diminishes, so that the cells approximate their lateral faces continually more closely. The secretion of fluid into the interior of the sphere and the resulting increase of the outer surface results from the characters of the whole wall, and cannot be explained by single, specially determined cells.
Finally, to take the case of the special kinds of blastospheres (_e.g._, of amphioxus, amphibia, reptiles, birds, and so forth), it has been already shown that these are produced by the shape of the egg, by the bulk of the yolk, and by the segregation of the yolk-particles under the influence of gravity; that, in fact, the shapes are determined by the general gross conditions of the structure of the egg.
Plainly, the blastosphere cannot be pre-existing as a structure of particles in the fertilised nucleus; there cannot be blastosphere determinants. The conditions for the origin of the blastosphere come into existence only by the process of segmentation, and it is only by its capacity to divide that the egg contains the conditions for blastosphere formation. Here we have epigenesis--the appearance of a new formation, not the becoming visible of pre-existing complexity.
The conditions of gastrulation and of the formation of the germinal layers are similar. The invagination of the blastosphere comes about by the co-operation of all the cells of its wall, by local differences in the rates of growth in that wall, from dissimilarities in its curvature, from many causes which have not yet been sufficiently sought out and investigated. As cell division itself depends not upon special particles, but upon changes in the entire nuclear contents, it follows that the growth of the blastosphere-wall, which is merely the sum of the growth of all the cells in it, cannot be determined by special groups of determinants.
As an attempt to explain gastrulation, the origin of the germinal layers and many other events of development, the doctrine of determinants has reversed cause and effect. Certain cells do not become invaginated into the segmentation cavity because they possess groups of determinants that impel them to the assumption of inner layer characters. The reverse is the truth.
Local conditions of growth cause the invagination of a set of the cells of the blastosphere-wall. This invaginated layer of cells, brought into a new position with regard to its environment, becomes the endoderm and receives the stimulus to assume the character appropriate to the new environment. It is unlogical to speak of endoderm in the fashion of many textbooks and treatises on embryology, while the so-called endoderm cells still form part of the outer surface of the blastosphere, or even while they are still in process of formation by cleavage. For 'inner germinal layer' implies a condition of position which is created by the invagination.
In fact, it is impossible, in thinking of the gastrula as in thinking of the blastosphere, to conceive that in the egg, which is a simple cell, there can be preformed by material particles in the nucleus a condition which implies the existence of two layers of cells.
Thus analysis of a special case leads to the same conclusion as is reached by the general reasoning of the earlier part of this section.
FOOTNOTES:
[7] _The Germplasm_, pp. 68, 69.
[8] The following treatises contain criticisms of Weismann's theories: W.
Haacke, _Gestaltung und Vererbung_; Leipzig, 1893; Herbert Spencer, articles in _Contemporary Review_ (1893-94); Romanes, _An Examination of Weismannism_; Longmans, 1893.
[9] Notwithstanding the objections raised by Bergh, Verworn, and Haacke, I abide by the supposition that the nucleus of reproductive cells contains the hereditary mass or germinal material. My reasons may be found in my text-book on _The Cell_ (English edit., p. 274). Briefly they are: 1. The equivalence of the male and female hereditary masses. 2. The equal distribution of the growing nuclear mass of the primary egg-cell among the daughter-cells that, arising from it, build up the organism. 3. The preservation of a constancy of bulk of the hereditary mass when fertilization occurs. 4. The isotropism of protoplasm. Following Pfluger, I mean by isotropism that the protoplasm of the egg does not contain local areas for the formation of different organs; but that, according to the conditions, any part of the protoplasm may be employed in the formation of any organ. Isotropism is merely the negation of His' doctrine of the presence of local areas for definite organs, and without losing its meaning, is compatible with the fact that many eggs have their poles different, and that others have a bilateral symmetry which determines the plane of the first division. 5. The fact that the first stages of many embryonic developments consist in the multiplication of the nuclear material and its distribution in the yolk, following which the yolk-mass cleaves into cells.
[10] English edition, p. 32.
[11] English edition, p. 34.
[12] In this section upon heteromorphosis I rely upon the following treatises, which have appeared recently. Loeb, _Untersuchungen zur physiologischen Morphologie der Thiere. Organbildung und Wachsthum_. Heft, 1 and 2 (1891-1892). H. de Vries, _Intracellulare Pangenesis_ (1889). H.
Driesch, _Entwicklungsmechamische Studien_, i.-vi.; _Zeitschrift f.
wissenschaft, Zool._, vol. liii.-lv. The same, _Zur Theorie der thierischen Formbildung._ _Biol. Centralblatt_, vol. xiii., 1893. Chabry, _Contribution a l'embryologie normale et teratologique des Ascidies simples. Jour. de l'Anat. et de Physiol._ (1887). Wilson, _Amphioxus and the Mosaic Theory.
Journal of Morph._ (1893). See also _Anatomischer Anzeiger_ (1892).
[13] Roux tried to give experimental evidence in favour of his mosaic theory in a treatise _On the Artificial Productions of Half-Embryos by the Destruction of one of the first two Cleavage-Cells, and on the Reconstruction of the Lost Parts_. _Virchow's Archiv._, vol. cxiv., 1888.
Roux defends his mosaic theory against Driesch and myself in (1) _Ueber das entwicklungsmechanische Vermogen jeder der beiden ersten Furchungszellen des Eies. Verhandl. der Anat. Gesellsch. der 6'ten Versamml. in Wien_, 1892. (2) _Ueber Mosaikarbeit und neuere Entwicklungshypothesen._ Anatomische Hefte von Merkel und Bonnet (1893). Also in _Biol.
Centralblatt_ (1893); in the _Anatom. Anzeiger_ (1893), and in the treatise _Die Methoden zur Erzeugung halber Froschembryonen und zum Nachweis der Beziehung der ersten Furchungsebenen des Froscheies zur Medianebene des Embryo. Anatom. Anzeiger._ (1894); Nos. 8 and 9.
If, as would appear from the last treatise, Roux would avoid being reckoned with evolutionists, he must abandon his mosaic theory, and this he has not done. I think in the present essay, on theoretical and experimental grounds I have shown the untenability of Roux's mosaic theory.
[14] The terms vertical and horizontal refer to the vertical axis of the egg, which passes through the animal and vegetative poles.--_Translator's note._
[15] Further details concerning these experiments may be found in HERTWIG, _Ueber den Werth der ersten Furchungszellen fur die Organbildung des Embryo_. Experimentelle Studien am Froschund Tritonei. _Archiv. fur Mikrosk. Anatomie_, vol. xlii., 1893, p. 710; Plate xli.; Figs. 1, 2, 27.
[16] For the facts in this section I rely in particular upon the writings of Vochting, Bert, Ollier, Trembley, Landois, Ponfick, and others:
H. VoCHTING: _Ueber Transplantation auf Pflanzenkorper_. _Untersuchungen zur Physiologie und Pathologie_; Tubingen, 1892.
VON GaRTNER: _Versuche und Beobachtungen ueber die Bastarderzeugung im Pflanzenreich_, 1849.
LeOPOLD OLLIER: _Recherches experimentales sur la production artificielle des os au moyen de la transplantation du perioste, etc._ _Journal de la physiologie de l'homme et des animaux_, tom. ii., 1859, pp. 1, 169, 468.
LeOPOLD OLLIER: _Recherches experimentales sur les greffes osseuses_. The same, tom. iii., p. 88, 1860.
PAUL BERT: _Recherches experimentales pour servir a l'histoire de la vitalite propre des tissus animaux_. _Annales des Sciences naturelles, Ser.
V., Zoologie_, tom. v., 1886.
VON RECKLINGHAUSEN: _Die Wiedererzeugung (Regeneration) und die Ueberpflanzung (Transplantation)_. _Handbuch d. Allgem. Pathologie des Kreislaufs aus Deutsche Chirurgie_, 1883.
TREMBLEY: _Memoires pour servir a l'histoire d'un genre de Polypes d'eau douce_, 1744.
LANDOIS: _Die Transfusion des Blutes_; Leipzig, 1875.
ADOLF SCHMITT: _Ueber Osteoplastik in klinischer und experimenteller Beziehung_. _Arbeiten aus der chirurgischenklinik der Konigl. Universitat, Berlin._
PONFICK: _Experimentelle Beatrage zur Lehre von der Transfusion_.
_Virchow's Archiv._, vol. lxii.
BERESOWSEY: _Ueber die histologischen Vorgange bei der Transplantation von Hautstucken auf Thiere einer anderen Species_. _Ziegler's Beitrage zur pathologischen Anatomie und zur allgemeinen Pathologie_; Jena, 1893.
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.