MATURATION OF THE OVUM. 75 



vesicle can be detected in the ovum, but outside it, close to the point where 

 the modified remnants of the vesicle were previously situated, there is 

 present a polar body which is composed of two parts, one of which stains 

 deeply and resembles the nuclear body, and the other does not stain but is 

 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 becom- 

 ing 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 pro- 

 bably 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 Arthropoda 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 socialis 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 ovum of Balanus 

 balanoides, but his observations are not perfectly satisfactory. 



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

 recently found by Grobben (" Entwicklungsgeschichte d. Moina rectirostris." Claus' 

 Arbeiten, Vol. II., Wien, 1879) near the surface of the protoplasm at the animal pole 

 of the summer and parthenogenetic eggs of Moina rectirostris, one of the Cladocera. 

 The body stains deeply with carmine, but differs from normal polar cells in not being 

 separated from the ovum ; and its identification as a polar cell must remain doubtful 

 till it has been shewn to originate from the germinal vesicle. 



2 "Zur Entwicklung d. Entomostraken." Niederlandischer Archiv. f. Zoologie, 

 Vol. m. p. 62. 



