THEIR SIGNIFICANCE IN HEREDITY. 383 



VI. RECAPITULATION. 



To bring- tog-ether shortly the results of this essay: the funda- 

 mental fact upon which everything- else is founded is the fact that 

 two polar bodies are expelled, as a preparation for embryonic develop- 

 ment, from all animal eg-gs which require 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 parthenogenetic 



egg". 



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 egg, 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 egg requiring fertilization, and the beginning 

 of embryogenesfis immediately follows fertilization. The partheno- 

 genetic 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 ' Immediately after the expulsion of the polar 

 body.' 



