MATURATION AND IMPREGNATION OF THE OVUM. 527 



The following is the summary which he gives of his results, 

 PP- 3578. 



" At the time when the egg is mature the germinal vesicle 

 undergoes a retrogressive metamorphosis and becomes carried 

 towards the surface of the egg by the contraction of the proto- 

 plasm. Its membrane becomes dissolved and its contents dis- 

 integrated and finally absorbed by the yolk. The germinal spot 

 appears, however, to remain unaltered and to continue in the 

 yolk and to become the permanent nucleus of the ripe ovum 

 capable of impregnation." 



After the publication of Butschli's monograph, O. Hertwig (12) 

 continued his researches on the ova of Leeches (Hcemopis and 

 Nephelis], and not only added very largely to our knowledge of 

 the history of the germinal vesicle, but was able to make a very 

 important rectification in Butschli's conclusions. The following 

 is a summary of his results : The germinal vesicle, as in other 

 cases, undergoes a form of degeneration, though retaining its 

 central position ; and the germinal spot breaks up into frag- 

 ments. The stages in which this occurs are followed by one 

 when, on a superficial examination, the ovum appears to be 

 absolutely without a nucleus ; but there can be demonstrated by 

 means of reagents in the position previously occupied by the 

 germinal vesicle a spindle nucleus with the usual suns at its 

 poles, which Hertwig believes to be a product of the fragments of 

 the germinal spot. This spindle travels towards the periphery of 

 the ovum and then forms the spindle observed by Butschli. At 

 the point where one of the apices of the spindle lies close to the 

 surface a small protuberance arises which is destined to form the 

 first polar cell. As the protuberance becomes more prominent 

 one half of the spindle passes into it. The spindle then divides 

 in the normal manner for nuclei, one half remaining in the pro- 

 tuberance, the other in the ovum, and finally the protuberance 

 becomes a rounded body united to the egg by a narrow stalk. 

 It is clear that if, as there is -every reason to think, the above 

 description is correct, the polar cell is formed by a simple pro- 

 cess of cell-division and not, as Butschli believed, by the forcible 

 ejection of the spindle. 



The portion of the spindle in the polar cell becomes a mass 

 of granules, and that in the ovum becomes converted without 



