CHAPTEE III 

 DIFFERENTIATION 



WE shall be obliged to limit ourselves to a discussion of the 

 internal factors. These are to be sought, firstly, in the initial 

 structure of the germ, that is, of the fertilized ovum, and, 

 secondly, in the interactions of the developing parts. 



I. The ovum comprises the cytoplasm and the nucleus, and 

 each of these constituents has its part to play in the determi- 

 nation of inheritable characters. 



A. THE CYTOPLASM. 



The evidence which we have just been reviewing has shown 

 us not only that the division of the nucleus is not qualitative, 

 but also that the cytoplasm is not the homogeneous isotropic 

 body imagined by the ' Mosaik-theorie ', at least as originally 

 propounded. We know now that the different parts of the 

 cytoplasm are related causally to the formation of the various 

 organs of the embryo, are therefore factors in differentiation 

 and determinants of inheritance. 



The testimony of experiment is conclusive on this point, 

 since it is known that the abstraction of a certain part of the 

 cytoplasm involves the absence of certain embryonic organs. 

 As the facts are fairly well known they need only be briefly 

 recapitulated here. 



The development of experimentally isolated parts of the 

 ovum they may be pieces of the unsegmented ovum, or blasto- 

 meres removed during cleavage has been observed in a 

 number of forms, and it has been shown that while it is 

 possible in certain cases to obtain a whole embryo or larva 

 from such a portion, the capacity of a part to develop into 

 a whole is conditioned by the structure of the egg-cytoplasm. 

 Thus in the Hydromedusae among the Coelenterates this cyto- 



