IV.] FOUNDATION OF A THEORY OF HEREDITY. 223 



many eggs that it was a long time before the cellular nature of 

 these structures was recognized ^ ; and it is possible that their 

 minute size may point to the fact that a phyletic process of 

 reduction has taken place, to the end that the Qgg may be 

 deprived of as little material as possible. It is at all events 

 proved that in all Metazoan groups the nucleus undergoes 

 changes during the maturation of the ^^^g, which are entirely 

 similar to those which lead to the formation of polar bodies in 

 those eggs which possess them. In the former instances it is 

 possible that nature has taken a shortened route to gain the 

 same end. 



It would be an important objection if it could be shown that 

 no process corresponding to the expulsion of polar bodies takes 

 place in the male germ-cells, for it is obvious that here also we 

 should, according to my theory, expect such a process to occur. 

 The great majority of sperm-cells differ so widely in character 

 from the ordinary indifferent (i.e. undifferentiated) cells, that 

 they are evidently histologically differentiated in a very high 

 degree, and hence the sperm-cells, like the yolk-forming germ- 

 cells, must possess a specific nuclear substance. The majority 

 of sperm-cells therefore resemble the somatic cells in that they 

 have a specific histological structure, but their characteristic 

 form has nothing to do with their fertilizing power, viz. with 

 their power of being the bearers of germ-plasm. Important as 

 this structure is, in order to render it possible that the egg-cell 

 may be approached and penetrated, it has nothing to do with 

 the property of the sperm-cell to transmit the qualities of the 

 species and of the individual to the following generation. The 

 nuclear substance which causes such a cell to assume the 

 appearance of a thread, or a stellate form (in Crustacea), or 

 a boomerang form (present in certain Daphnids), or a conical 

 bullet shape (Nematodes), cannot possibly be the same nuclear 

 substance as that which, after conjugation with the egg-cell, 

 contains in its molecular structure the tendency to build up a 

 new Metazoon of the same kind as that by which it was pro- 

 duced. We must, therefore, conclude that the sperm-cell also 

 contains two kinds of nucleoplasm, namely, germ-plasm and 

 spermogenetic nucleoplasm. 



^ Van Beneden, even in his last work, considers these bodies to have 

 only the value of nuclei j 1. c, p. 394. 



