526 MATURATION AND IMPREGNATION OF THE OVUM. 



respect his observations are of value as tending to shew the 

 wide-spread existence of these structures. Negative results with 

 reference to the presence of the polar bodies have, it may be re- 

 marked, only a very secondary value. Biitschli has made the 

 very important discovery that in perfectly ripe eggs of Nephelis, 

 Lymnaeus and Cucullanus and allied genera a spindle, similar to 

 that of ordinary nuclei in the act of division, appears close to 

 the surface of the egg. This spindle he regards as the meta- 

 morphosed germinal vesicle, and has demonstrated that it takes 

 part in the formation of the polar cells. He states that the 

 whole spindle is ejected from the egg, and that after swelling up 

 and forming a somewhat spherical mass it divides into three parts. 



In the Nematodes generally. Biitschli has been unable to find 

 the spindle modification of the germinal vesicle, but he 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. This de- 

 scription of Biitschli resembles in some respects that given by 

 van Beneden of the changes in the rabbit's ovum, and not im- 

 possibly refers to a nearly identical series of phenomena. The 

 discovery by Biitschli of the spindle and its relation to the polar 

 body has been of very great value. 



The publications of van Beneden, and more especially those 

 of Biitschli, taken by themselves lead to the conclusion that the 

 whole germinal vesicle is either ejected or absorbed. Nearly 

 simultaneously with their publications there appeared, however, 

 a paper by Oscar Hertwig (n) on the eggs of one of the com- 

 mon sea urchins (Toxopneustes lividus), in which he attempted to 

 shew that part of the germinal vesicle, at any rate, was con- 

 cerned in the formation of the first segmentation nucleus. He 

 believed (though he has himself now recognised that he was in 

 error on the point) that no polar cell was formed in Toxop- 

 neustes, and that the whole germinal vesicle was absorbed, with 

 the exception of the germinal spot which remained in the egg as 

 the female pronucleus. 



