240 Artificial Parthenogenesis and Fertilization 



the egg permanently and which saves it from the disintegra- 

 tion which follows membrane forrhation. 



3. In the preceding experiment blastomeres were fertilized 

 which had ceased to segment for about twenty hours. The 

 experiment leads, however, to an entirely different result if 

 sperm is added while the blastomeres are in active partheno- 

 genetic cell division. If we add sperm to such eggs {S. purpuror 

 tus), they form a fertilization membrane, but they do not 

 develop very far. The entrance of a spermatozoon into the 

 blastomere of an egg which is in active parthenogenetic seg- 

 mentation leads to the rapid disintegration of the egg or blas- 

 tomere ; while the entrance of a spermatozoon into a partheno- 

 genetic blastomere which has gone back into the resting stage 

 for some time, can cause the development of the blastomere 

 into a normal pluteus. What causes this difference ? Possibly 

 the fact that the blastomere which had gone back into the rest- 

 ing stage for some time has lost centrosomes and astrospheres, 

 while the egg which is in active parthenogenetic cell division 

 possesses both organs. 



These blastomeres in which fertilization by sperm is super- 

 imposed upon artificial parthenogenesis while the eggs are 

 still in active development behave like eggs fertilized by more 

 than one spermatozoon. Driesch found that eggs which had 

 been fertilized by more than one spermatozoon do not, for the 

 most part, develop beyond the blastula stage.' Boveri has 

 explained this by the fact that such an egg possesses more 

 than two astrospheres.^ As we know, the division of the 

 nucleus into two daughter nuclei depends upon the fact that 

 the dividing egg forms two astrospheres. This is the case 

 not only in fertilization by sperm, but also in the development 

 started by the methods of artificial parthenogenesis. But 



1 Driesch, " Ueber die Purcliiing doppelbefruchteter Eier," Zeilschr. /. 

 wissenschft. Zool., LV, 1892. 



2 Boveri, Zellenstudien, Heft 6; Die Entwicklung dispermer SeeigeUier Leipzig 

 1907. 



