62 FUNCTION OF POLAR CELLS. 



analogous process amongst plants. Two questions about it obviously 

 present themselves for solution : (1) 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 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 necessary from the thick- 

 ness 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 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 Biitschli, takes place during conjugation 

 in Infusoria as a preliminai-y to the formation of a fresh nucleus. 

 This comparison is due to Biitjschli, 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 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, 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 formation 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), 



