98 THE BIOLOGICAL PROBLEM OF TO-DA Y 



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 ceils 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 forma- 

 tion. Here we have epigenesis the appearance of 

 a new formation, not the becoming visible of pre- 

 existing complexity. 



