VI.] THEIR SIGNIFICANCE IN HEREDITY. 395 



fertilization, while only one such body is expelled from all 

 parthenogenetic eggs. 



This fact in the first place refutes every purely morphological 

 explanation of the process. If it were physiologically valueless, 

 such a phyletic reminiscence of the two successive divisions of 

 the egg-nucleus must have been also retained by the partheno- 

 genetic ^^g. 



In my opinion the expulsion of the first polar body implies 

 the removal of ovogenetic nucleoplasm when it has become 

 superfluous after the maturation of the egg has been completed. 

 The expulsion of the second polar body can only mean the 

 removal of part of the germ-plasm itself, a removal by which 

 the number of ancestral germ-plasms is reduced to one half. 

 This reduction must also take place in the male germ-cells, 

 although we are not able to associate it confidently with any of 

 the histological processes of spermatogenesis which have been 

 hitherto observed. 



Parthenogenesis takes place when the whole of the ancestral 

 germ-plasms, inherited from the parents, are retained in the 

 nucleus of the egg-cell. Development by fertilization makes 

 it necessary that half the number of these ancestral germ- 

 plasms must be first expelled from the ^g^-i the original quantity 

 being again restored by the addition of the sperm-nucleus to 

 the remaining half 



In both cases the beginning of embryogenesis depends upon 

 the presence of a certain, and in both cases equal, quantity of 

 germ-plasm. This certain quantity is produced by the addition 

 of the sperm-nucleus to the ^gg requiring fertilization, and the 

 beginning of embryogenesis immediately follows fertilization. 

 The parthenogenetic egg contains within itself the necessary 

 quantity of germ-plasm, and the latter enters upon active 

 development as soon as the single polar body has removed the 

 ovogenetic nucleoplasm. The question which I have raised on 

 a previous occasion — ' When is the parthenogenetic egg capable 

 of development ? ' — now admits of the precise answer — ' Im- 

 mediately after the expulsion of the polar body.' 



From the preceding facts and considerations the important 

 conclusion results that the germ-cells of any individual do not 

 contain the same hereditary tendencies, but are all different, in 

 that no two of them contain exactly the same combinations of 



