MATURATION AND IMPREGNATION OF THE OVUM. 113 



concluding that the formation of the polar cells is connected with 

 the disappearance, partial or otherwise, of the germinal vesicle. A 

 very similar account of the apparent disappearance of the ger- 

 minal vesicle is given by GreefF (19) who states that the apparent 

 disappearance of the germinal spot precedes that of the vesicle. 



The observations of Biitschli are of still greater importance in 

 this direction. He has studied with a view to elucidating the 

 fate of the germinal vesicle, the eggs of Nephelis, Lymnseus, 

 Cucullanus, and other Nematodes ; and Eotifers. In all of these, 

 with the exception of Rotifers, he finds polar bodies, and in this 

 respect his observations are of value as tending to show 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, 

 Lymnseus 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 ^^^, 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 t^g. When it is 

 in contact with the surface a small spherical body, the remnant 

 of 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 

 impossibly 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 pubhcations 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 (11) on the eggs of one of the common 

 sea urchins [Toxopneustes Uvidus) ,m which he attempted to show 

 that part of the germinal vesicle, at any rate, was concerned in 

 the formation of the first segmentation nucleus. He believed 

 (though he has himself now recognised that he was in error on 



