360 ON THE NUMBER OF POLAR BODIES AND [VI. 



the maturation of the eggs of insects ; and it is a curious ill-luck 

 that he, like many other observers, did not succeed in observing 

 the expected expulsion of polar bodies, in spite of the great 

 trouble he had taken. It may be that the species selected for 

 investigation were unfavourable : at all events, we cannot now 

 doubt that a division of the egg-nucleus is quite universal 

 among insects, for Blochmann, in his latest contribution to the 

 subject, proves that the Aphidae also form polar bodies. He 

 examined the winter-eggs oi Aphis aceris, and ascertained that 

 they form two polar bodies, one after the other. Even in the 

 viviparous Aphidae, thin sections revealed the presence of a 

 polar body, though Blochmann could not trace all the stages of 

 its development. It appears that the polar body is here pre- 

 served for an exceptional period, and its presence can still be 

 proved when the blastoderm has been formed, and sometimes 

 when development is even further advanced. Skilled observers 

 of recent times, such as Will and Witlaczil, have not been able 

 to find a polar body in the parthenogenetic eggs of the Aphidae, 

 and Blochmann's proof of its existence seems to me to be of 

 especial value, because the eggs o^ Aphidae are in many respects 

 so unusually reduced ; for instance, the primary yolk is absent 

 and the egg-membrane is completely deficient, so that w^e 

 might have expected that if polar bodies are ever absent, they 

 would be wanting in these animals — that is, if they were of no 

 importance, or at any rate of only secondary importance. 



Hence the presence of polar bodies in Aphidae is a fresh con- 

 firmation of their great physiological importance. As bearing 

 upon the main question dealt with in this essa}^, Blochmann's 

 observations have an especial interest, because only one polar 

 body was found in the parthenogenetic eggs o^ Aphis, while the 

 sexual eggs normally produce two. The author rightly states 

 that this result is in striking accordance with my results obtained 

 from the summer-eggs of different Daphnidae, and he adds the 

 remark, — ' It would be of great interest to know whether these 

 facts are due to the operation of some general law.' To this 

 remark I can now reply that there is indeed such a law : not 

 only in the parthenogenetic eggs of Daphnidae, but also, as I 

 have since found, in those of the Ostracoda and Rotifera^, only 



^ In the summer-eggs of Rotifera I have, together with Mr. Ischikawa, 

 observed one polar body, and we were able to establish for certain that 



