N ON -MOSAIC THEORIES 421 



We may suppose that, from the youngest ovarian ovum onwards, 

 the nucleus exerts a " control " upon the surrounding cytoplasm, 

 whether by the migration of " pangens " (De Vries), or of 

 specific formative substances (Sachs), or of enzymes, or by a 

 propagation of molecular movements (Nageli). In some such 

 way — varying greatly in degree in different cases — the nucleus 

 prepares in the cytoplasm of the egg a framework for its future 

 operations. This may be so slightly pre-established that from 

 a minute fragment of the egg a complete larva may be reared 

 (as in sea-urchins), or so well established that if a part of the 

 unsegmented egg be removed the remainder forms a defective 

 larva (ctenophore). 



The nucleus of the fertilised egg-cell divides into equivalent 

 halves, but these find themselves in more or less different territory, 

 as the result of the preparatory framework which the nucleus, 

 before division, had established in the cytoplasm. In technical 

 language, the nuclei, though equivalent, find themselves in a 

 not altogether isotropic medium. 



The dividing nuclei, as Driesch and Boveri suggest, are differ- 

 ently stimulated to expression in the different areas of the hetero- 

 geneous cytoplasm, and they thus call forth new differentiations 

 in these, in ever-increasing complexity of action and reaction. 



If the initial cytoplasmic differentiation was slight, the first 

 steps in differentiation will be correspondingly slight, and in these 

 cases an isolated cleavage-cell or blastomere may still form a 

 complete embryo, as in the lancelet. If the initial cytoplasmic 

 differentiation was more pronounced, an isolated blastomere may 

 not be able to do more than form a partial embryo ; the "setting " 

 of the cytoplasm may be too strong to be overcome even by the 

 completely equipped blastomere-nucleus. 



Thus we reach the idea, expressed, for instance, by Driesch, 

 that " the relative position of a blastomere in the whole deter- 

 mines in general what develops from it ; if its position be changed, 

 it gives rise to something different ; in other words, its prospective 



