HEREDITY 



265 



the sperm. The cytoplasm of the egg responds to the stimulus 

 produced by the entrance of the sperm by a rearrangement of 

 its substance to produce the gray crescent, which is the first 

 indication of bilateral symmetry. Certainly the cytoplasm is 

 organized and this organization is inherited, but for reasons to be 

 considered later it is difficult to express the basis of this organiza- 

 tion in terms of hereditary factors. In general terms it might be 

 said that the cytoplasm of the egg is concerned with the deter- 

 mination of the groundwork on which the chromosomal factors 

 operate. It is to be noted that there is no contribution from the 

 sperm comparable to the cytoplasm of the egg. The latter is a 

 purely maternal contribution to the development of the embryo. 

 The conclusion is justified that there is in the egg cytoplasm a 



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Fig. 162. — Diagram to illustrate Weismann's germ track, e, fertilized egg; 



g, germ cell; s, soma. 



specific and highly organized substance, which with the chromo- 

 somes constitutes the material basis of development and heredity. 

 Inheritance of Acquired Characters. — In many animals the 

 primordial germ cells are segregated from the somatic cells at a 

 relatively early stage in ontogeny. Both germ cells and somatic 

 cells are derived from the fertilized egg, but the germ cells retain 

 the reproductive potency of the egg while the somatic cells lose 

 it, presumably because the latter become differentiated into the 

 various tissues of the body. It is generally believed that in 

 higher forms the germ cells of the individual come only from the 

 primordial germ cells and not from somatic cells. The reproduc- 

 tive potencies are therefore retained by the germ cells simply 

 because they do not undergo differentiation into tissue cells. 

 The somatic cells die with the individual, but descendants of the 

 germ cells live on in following generations forming a line of 

 germinal continuity or germ track (Fig. 162) connecting genera- 



