230 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



polar body is the male part of the hermaphrodite egg-cell. An 

 ^gg which has lost its male part cannot develope into an embryo 

 until it has received a new male part in fertilization. On the 

 other hand, an egg which does not expel its male part may 

 develope without fertilization, and thus we are led to the 

 obvious conclusion that parthenogenesis is based upon the 

 non-expulsion of polar bodies. Balfour distinctly states 'that 

 the function of forming polar cells has been acquired by the 

 ovum for the express purpose of preventing parthenogenesis \' 



It is obvious that I cannot share this opinion, for I regard 

 the expulsion of polar bodies as merely the removal of the 

 ovogenetic nucleoplasm, on which depended the development 

 of the specific histological structure of the egg-cell. I must 

 assume that the phenomena of maturation in the partheno- 

 genetic egg and in the sexual egg are precisely identical, and 

 that in both, the ovogenetic nucleoplasm must in some way be 

 removed before embryonic development can begin. 



Unfortunately the actual proof of this assumption is not so 

 complete as might be desired. In the first place, we are as 

 yet uncertain whether polar bodies are or are not expelled by 

 parthenogenetic eggs ^ ; for in no single instance has such ex- 

 pulsion been established beyond doubt. It is true that this 

 deficiency does not afford any support to the explanation of 

 Minot and Balfour, for in all cases in which polar bodies have 

 not been found in parthenogenetic eggs, these structures are 

 also absent from the eggs which require fertilization in the 

 same species. But although the expulsion of polar bodies in 

 parthenogenesis has not yet been proved to occur, we must 

 assume it to be nearly certain that the phenomena of matura- 

 tion, whether connected or unconnected with the expulsion of 

 polar bodies, are the same in the eggs which develope par- 

 thenogenetically and in those which are capable of fertilization, 

 in one and the same species. This conclusion depends, above 

 all, upon the phenomena of reproduction in bees, in which, as 

 a matter of fact, the same egg may be fertilized or may develope 

 parthenogenetically, as I shall have occasion to describe in 

 greater detail at a later period. 



^ F. M. Balfour, ' Comparative Embryology,' vol. i. p. 63. 



^ The formation of a polar body in parthenogenetic eggs has now 

 been proved : see note at the end of this Essay ; see also Essay VI. — 

 A. W., 1888. 



