114 F. M. BALFOUR. 



the point) that no polar cell was formed in Toxopneustes, and that 

 the whole germinal vesicle was absorbed, with the exception of the 

 germinal spot which remained in the egg as the female pronucleus. 



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

 pp. 357-8. 



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

 dergoes 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 disin- 

 tegrated 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 Biitschli's monograph, O. Hertwig (12) 

 continued his researches on the ova of Leeches (^llcemopis and 

 Nephel'is), 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 Biitschli'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 fragments. 

 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 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 Biitschli. At the point where one 

 of the apices of the spindle lies close to the surface a small pro- 

 tuberance 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 protuberance, 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 simple process of cell- division and 

 not, as Biitschli believed, by the forcible ejection of tiie spindle. 



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

 granules, and that in the ovum becomes converted without the 

 occurrence of the usual nuclear stage into a fresh spindle. A 

 second ])olar cell is formed in the same manner as the first one, 

 and the first one subsequently divides into two. The portion of 

 the spindle which remains in the egg after the formation of the 

 second polar cell reconstitutes itself into a nucleus — tiie female 



