(1) In the division of the egg-cell cavities arise between the four, eight, and sixteen pieces, and thus the whole contents of the egg become arranged more loosely. (2) The more the cells multiply by division and become smaller in circumference, the more closely they apply their lateral surfaces to each other, especially at the outer surface of the whole, so assuming the arrangement of cell-epithelia. (3) By the secretion of fluid, a constantly growing central cavity is formed pari passu with the approximation of the superficial cells, and this probably also brings with it an increase of the internal pressure, and a wider curvature of the wall of the sphere.
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.