FOUNDATION OF A THEORY OF HEREDITY. 219 



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 ordi- 

 nary 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 produced. We must, therefore, conclude that the 

 sperm-cell also contains two kinds of nucleoplasm, namely, germ- 

 plasm and spermogenetic nucleoplasm. 



It is true that we cannot say a priori whether the influence 

 exercised on the sperm-cell by the spermogenetic nucleoplasm 

 might not be eliminated by some means other than its removal 

 from the cell. It is conceivable, for instance, that this substance 

 may be expelled from the nucleus, but may remain in the cell-body, 

 where it is in some way rendered powerless. We do not yet really 

 know anything of the essential conditions of nuclear division, and 

 it is quite impossible to bring forward any facts in svipport of my 

 previous suggestion. The germ-plasm is supposed to be present 

 in the nucleus of the growing egg-cell in smaller quantity than the 

 ovogenetic nucleoplasm, and the germ-plasm gradually increases in 

 quantity: thus when the egg has attained its maximum size, the 

 opposition between the two different kinds of nucleoplasm becomes 

 so marked, in consequence of the alteration in their quantitative 

 relations, that their separation, viz. nuclear division, results. But 



