Part 6 (1/2)

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 a.s.sociated with separate, special material bearers. The appearance of s.p.a.ces 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 a.s.sumption of a spherical shape--an a.s.sumption 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 inv.a.g.i.n.ation 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 inv.a.g.i.n.ated into the segmentation cavity because they possess groups of determinants that impel them to the a.s.sumption of inner layer characters. The reverse is the truth.

Local conditions of growth cause the inv.a.g.i.n.ation of a set of the cells of the blastosphere-wall. This inv.a.g.i.n.ated layer of cells, brought into a new position with regard to its environment, becomes the endoderm and receives the stimulus to a.s.sume the character appropriate to the new environment. It is unlogical to speak of endoderm in the fas.h.i.+on 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 inv.a.g.i.n.ation.

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 a.n.a.lysis 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 ma.s.s 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 ma.s.ses. 2. The equal distribution of the growing nuclear ma.s.s 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 ma.s.s 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-ma.s.s 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 pa.s.ses 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.