36 THE PHYSICAL BASIS OF LIFE 



were contained in the nucleus and that the cytoplasm or 

 ooplasm is indifferent in character. This conception was 

 overthrown by experiments which demonstrated that l^y 

 subjecting the egg to pressure during cleavage the rela- 

 tive positions of the nuclei might be widely altered with- 

 out materially affecting the cytoplasmic materials; yet 

 such eggs, when released from pressure, often produce 

 nearly or quite normal eml^ryos.'" It must therefore be cy- 

 toplasmic materials, rather than nuclear, that undergo a 

 differential distribution by cleavage — a result supported 

 hj numerous cytological observations which have shown 

 that the cytoplasmic components of different cells often 

 show characteristic diiferences made visible by the pres- 

 ence of pigment, of specific types of granulation, or other 

 characters. 



The conception of an original germinal prelocalization 

 in the cytoplasm seems at first sight to be supported by 

 the fact that these different ooplasmic materials are often 

 visible as such in the egg before cleavage begins, often be- 

 fore it is fertilized ; and especially by the fact that even at 

 this time they sometimes display a definite structural pat- 

 tern which in certain important respects already offers a 

 plan or outline of the coming embryo. The germ-forming 

 regions or ooplasmic areas thus indicated are not marked 

 off by sharply defined or fixed boundaries prior to cleav- 

 age, but as this process proceeds they become clearly de- 

 fined by cleavage-planes which isolate their materials in 

 particular cells, or groups of cells, of the early embryo. 

 A well-known example of these facts is offered by the sea- 

 urchin Paracentrotus lividus in which, as demonstrated 

 by Boveri, the egg at a certain period prior to its fertiliza- 

 tion shows three horizontal zones (Fig. 15, 16), an upper 

 clear one which produces the ectoderm of the larva, a 

 middle red-pigmented one that gives rise to the entoderm 



