MATURATION OF THE OVUM. 77 



bodies by a process 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 Butschli, takes place during conjugation in In- 

 fusoria as a preliminary to the formation of a fresh nucleus. 

 This comparison is due to Butschli, and according to it the 

 formation of the polar bodies would have to be regarded as 

 assisting, in some as yet unknown way, the process of regene- 

 ration 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, is 

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

 it again by the spermatic nucleus. 



My view amounts to the following, viz. that after the forma- 

 tion of the polar cells the remainder of the germinal vesicle 

 within the ovum (the female pronucleus) is incapable of further 

 development without the addition of the nuclear part of the 

 male element (spermatozoon), and that if polar cells were not 

 formed parthenogenesis might normally occur. A strong sup- 

 port for this hypothesis would be afforded were it to be definitely 

 established that a polar body is not formed in the Arthropoda 

 and Rotifera ; since the normal occurrence of parthenogenesis 

 is confined to these two groups. It is certainly a remarkable 

 coincidence that they are the only two groups in which polar 

 bodies have not so far been satisfactorily observed. 



It is perhaps possible that the part removed in the formation of the 

 polar cells is not absolutely essential ; and this seems at first sight to follow 

 from the fact of parthenogenesis being possible in instances where impreg- 

 nation is the normal occurrence. The genuineness of the observations 

 on this head is too long a subject to enter into here 1 , but after admitting, 



1 The instances quoted by Siebold, Parthenogenesis d. Arthropoden, are not quite 

 satisfactory. In Hensen's case, p. 234, impregnation would have been possible if we 

 can suppose the spermatozoa to be capable of passing into the body-cavity through the 



