MATURATION AND IMPREGNATION OF THE OVUM. 537 



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 regarded 

 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 neces- 

 sary 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 cells by a pro- 

 cess identical with cell division. The ejection of part of the 

 germinal vesicle in the formation of the polar cells may probably 

 be paralleled by the ejection of part or the whole of the original 

 nucleus which, if we may trust the beautiful researches of Biit- 

 schli, takes place during conjugation in Infusoria as a preliminary 

 to the formation of a fresh nucleus. This comparison is due to 

 Biitschli, and according to it the forma'tion of the polar bodies 

 would have to be regarded as assisting, in some as yet unknown 

 way, the process of regeneration of the germinal vesicle. Views 

 analogous to this are held by Strasburger and Hertwig, who 

 regard the formation of the polar bodies in the light of a process 

 of excretion or removal of useless material. Such hypotheses 

 do not unfortunately carry us very far. 



I would suggest that in the formation of the polar cells 

 part of the constituents of the germinal vesicle which are requisite 

 for its functions as a complete and independent nucleus are 

 removed to make room for the supply of the necessary parts to 

 it again by the spermatic nucleus (vide p. 541). More light on 

 this, as on other points, may probably be thrown by further 

 investigations on parthenogenesis and the presence or absence 

 of a polar cell in eggs which develope parthenogenetically. 

 Curiously enough the two groups in which parthenogenesis most 

 frequently occurs in the ordinary course of development (Arthro- 

 poda and Rotifera) are also those in which polar cells, with the 

 possible exception mentioned above, of the parthenogenetic eggs 

 of Lacenularia, are stated to be absent. This curious coincidence, 



B. 35 



