MATURATION OF THE OVUM. 61 



similar to the nucleoplasmic body. Van Beneden concludes that the parts of 

 the polar body are the two ejected products of the germinal vesicle. We may 

 be perhaps permitted to hold that further observations on this difficult object 

 will demonstrate that part of the germinal vesicle remains in the ovum to 

 form the female pronucleus. 



With reference to invertebrate forms attention may be called to the 

 observations of Biitschli (80). Although in Cucullanus a normal formation 

 of the polar bodies takes place, yet in the Nematodes generally, Biitschli has 

 been unable to find the spindle modification of the germinal vesicle, but 

 states that the germinal vesicle undergoes degeneration, its outline be- 

 coming indistinct and the germinal spot vanishing. The position of 

 the germinal vesicle continues to be marked by a clear space, which 

 gradually approaches the surface of the egg. When it is in contact with 

 the surface a small spherical body, the remnant of the germinal vesicle, comes 

 into view, and eventually becomes ejected. The clear space subsequently 

 disappears. 



In addition to the types just quoted, which may very probably turn 

 out to be normal in the mode of formation of the polar bodies, there is 

 a large number of types, including the whole of the Rotifera and 

 Arthropod a ; with a few doubtful exceptions 1 , in which the polar cells 

 cannot as yet be said to have been satisfactorily observed. 



The more important of the doubtful cases amongst the Rotifera and Ar- 

 thropoda are the following. 



Flemming (83) finds that in the summer and probably parthenogenetic 

 eggs of Lacinularia socialls the germinal vesicle approaches the surface 

 and becomes invisible, and that subsequently a slight indentation in 

 the outline of the egg marks the point of its disappearance. In the hollow 

 of the indentation Flemming believes a polar cell to be situated, though he 

 has not definitely seen one. 



Hoek 2 believes that he has found a polar body in the ovurn of Balanus 

 balanoides, but his observations are not perfectly satisfactory. 



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 investiga- 

 tion, 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 



1 The best instance of what appears like a polar cell in Arthropocla is a body recently 

 found by Grobben (" Eutwicklungsgescbicbte d. Moina rectirostris." Clans' Arbeit en, 

 Vol. ii., Wien, 1879) near the surface of tbe protoplasm at the animal pole of tbe summer 

 and partbenogenetic eggs of Moina rectirostris, one of the Cladocera. Tbe body stains 

 deeply with carmine, but differs from normal polar cells in not being separated from 

 tbe ovum ; and its identification as a polar cell must remain doubtful till it has been 

 shewn to originate from tbe germinal vesicle. 



2 " Zur Eutwicklung d. Eutomostraken." Niederlandischer Archiv. f. Zoologie, vol. 

 in. p. 62. 



