NON-MOSAIC THEORIES 425 



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). We may think of the controlling agency 

 of a new colony laying down some preliminary lines of exploita- 

 tion in the surrounding territory, making a railroad here, a canal 

 there, a mere telegraph line somewhere else, but not sending out 

 specialised settlers into the various areas, as the theory of differen- 

 tial divisions would suggest. 



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. This incites further dif- 

 ferentiation, both in the nuclear material and in the cytoplasmic 

 sphere of influence. 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 



