9 

 Parth en og en esis 



jDiological processes often reveal themseles as 

 themes with many variations. Fugitive incidental nuances 

 embellish the process of fertilization, as we have seen; but 

 though they run the whole range of variation, they never 

 obscure the motive: fertilization as the union of egg-plasma 

 and spermatozoon. 



That the egg- and sperm-nucleus are equipotent in the 

 developmental process, since with either alone the egg's 

 development can successfully proceed, has been pointed out. 

 Thus, whilst the development of the egg of any one of sev- 

 eral marine invertebrate animals can be initiated by fer- 

 tilizing it after its nucleus has been removed, the egg-nucleus 

 of the tg^ of Rhahditis aherrans alone takes part in devel- 

 opment, for the sperm-nucleus within the Q^g remains 

 Inert. Neither of these examples is a violation of the state- 

 ment that for the majority of multicellular animals the life 

 of the new individual begins with the coming together of 

 the two living gametes, egg and spermatozoon. Rather, 

 both demonstrate very clearly this essential fact: fertiliza- 

 tion Is not the fusion of the egg- and sperm-nuclei but the 

 union of egg-plasma and spermatozoon. Now we turn to 

 the discussion of phenomena which reveal that even this 

 union Is not always necessary for the elevation of the life- 

 process in the egg from the level of a single cell to that of 

 an individual of multicellular organization. There are 

 eggs which normally develop without spermatozoa. This 

 type of development, encountered among many animal 

 species. Is called parthenogenesis. Of the theme, the Inltia- 



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