116 



F. M. BALFOUR. 



polar cells, and states his belief that an equivalent of the polar 

 cells is widely distributed in the vegetable subkingdom. 



This sketch of the results of recent researches will, it is hoped, 

 suffice to bring into prominence the more important steps 

 by which the problems of this department of embryology have 

 been solved. The present aspects of the question may perhaps 

 be most conveniently displayed by following the history of a 

 single ovum. Tor this purpose the eggs of Asterias gla- 

 cialis, which have recently formed the subject of a series of 

 beautiful researches by Fol (9), may conveniently be selected. 



Fig. 2. — Ripe ovum of Asterias glaeialis enveloped in a mucilaginous en- 

 velope, and containing an excentric germinal vesicle and germinal 

 spot (copied from Fol). 



The ripe ovum (fig. 2), when detached from the ovary, is formed 

 of a granular vitellus without a vitelline membrane, but enveloped 

 in a mucilaginous coat. It contains an eccentrically situated 

 germinal vesicle and germinal spot. In the former is present 

 the usual protoplasmic reticulum. As soon as the ovum reaches 

 the sea water the germinal vesicle commences to undergo a 

 peculiar metamorphosis. It exhibits frequent changes of form, 



/?K 



Fig. 3. — Two successive stages in the gradual metamorphosis of the 

 gerniinal vesicle and spot of tlie uvnu ut AstcriaS glaCiali.i inune- 

 djalelj after it is laid (copied from Fol). 



