XII.] CONJUGATION AND SEXUAL REPRODUCTION. Ijl 



from the first an adaptation to fertilization, they have at any 

 rate the effect of checking the power of development in the 

 egg, so that, in a certain sense, we may maintain that their 

 expulsion prevents parthenogenesis. On the other hand, we 

 are now aware that a polar body is expelled from the partheno- 

 genetic egg, while the difference between this and the egg 

 requiring fertilization lies in the fact that a second polar body is 

 expelled from the latter; but the correct idea nevertheless 

 remains that something indispensable for the power of develop- 

 ment is removed from the egg. According to our present 

 views this is not the unknown ' male principle,' but a certain 

 quantity of germ-plasm. 



When we begin to enquire into the manner in which the 

 power of parthenogenetic development was gained by an egg 

 which required fertilization from the most remote time at which 

 multicellular beings existed, the first thought that strikes us is, 

 — might not this have been brought about by the suppression of the 

 second polar division ? If this happened, the first polar division 

 would cause a diminution to the normal number of the pre- 

 viously doubled idants, and the second polar division being 

 absent, the egg-cell would retain precisely as much nuclear 

 material as it would have contained if fertilization had followed 

 the expulsion of the second polar body. Since, then, regular 

 parthenogenetic eggs invariably possess only one polar body, 

 this supposition attains a high degree of probability. There are, 

 however, facts which show that parthenogenesis may be 

 acquired in another way. 



Blochmann has observed, as is well known, that when the 

 egg of the queen-bee is deposited in the cell of a drone, the 

 same course of maturation is pursued as when it is laid in a 

 female cell. In both cases two polar nuclei are formed, in both 

 the nuclear substance is halved twice successively. In the case 

 of the unfertilized male egg, the nucleus which remains after 

 the second division possesses the power of becoming the germ- 

 nucleus, and of developing ; while the female egg is only able 

 to enter upon embryogeny after the fusion of its nucleus with 

 that of the fertilizing spermatozoon. 



The eggs of Lepidoptera behave in a somewhat similar way ; 

 in the great majority of cases they require fertilization, but some 

 can develope parthenogenetically. In the case of Liparis dispar, 



