76 FUNCTION OF POLAR CELLS. 



Biitschli, who has expressly searched for the polar bodies in the ova of 

 Rotifera, was unable to find any trace of them, though he found that as the 

 egg became ripe the germinal vesicle became half its original size. In the 

 parthenogenetic eggs of Aphis he also failed to find a trace of polar bodies, 

 though the germinal vesicle, after the germinal spot had broken up into 

 fragments, approached the surface and disappeared. 



Whatever may be the eventual result of more extended 

 investigation, it is clear that the formation of polar cells 

 according to the type described above is a very constant 

 occurrence. Its importance is increased by the discovery by 

 Strasburger of the existence of an analogous process amongst 

 plants. Two questions about it obviously present themselves 

 for solution : (i) What are the conditions of its occurrence with 

 reference to impregnation ? (2) What meaning has it in the 

 development of the ovum or the embryo ? 



The answer to the first of these questions is not difficult to 

 find. The formation of the polar bodies is independent of 

 impregnation, and is the final act of the normal growth of the 

 ovum. In a few types the polar cells are formed while the 

 ovum is still in the ovary, as, for instance, in some species of 

 Echini, Hydra, etc., but, according to our present knowledge, far 

 more usually after the ovum has been laid. In some instances 

 the budding-off of the polar cells precedes, and in other instances 

 follows impregnation ; but there is no evidence to shew that in 

 the latter cases the process is influenced by the contact with the 

 male element. In Asterias, as has been shewn by O. Hertwig 

 and Fol, the formation of the polar cells may indifferently either 

 precede or follow impregnation a fact which affords a clear 

 demonstration of the independence of the two occurrences. 



To the second of the two questions it does not unfortunately 

 seem possible at present to give an answer which can be re- 

 garded as satisfactory. 



The retrogressive changes in the membrane of the germinal 

 vesicle which usher in the formation of the polar bodies may 

 very probably be viewed as a prelude to a renewed activity of 

 the contents of the vesicle ; and are perhaps rendered the more 

 necessary from the thickness of the membrane which results 

 from a protracted period of passive growth. This suggestion 

 does not, however, help us to explain the formation of polar 



