528 MATURATION AND IMPREGNATION OF THE OVUM. 



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

 second polar 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 the female 

 pronucleus and travelling towards the centre of the egg un- 

 dergoes a fate which will be spoken of in the second part of this 

 paper. 



The most obscure part of Hertwig's work is that which con- 

 cerns the formation of the spindle on the atrophy of the germinal 

 vesicle, and his latest paper, though it gives further details on 

 this head, does not appear to me to clear up the mystery. 

 Though Hertwig demonstrates clearly enough that this spindle 

 is a product of the metamorphoses of the germinal vesicle, he 

 does not appear to prove the thesis which he maintains, that it 

 is the metamorphosed germinal spot. 



Fol, to whom we are indebted in his paper on the develop- 

 ment of Geryonia (7) for the best of the earlier descriptions of 

 the phenomena which attend the maturation of the egg, and 

 later for valuable contributions somewhat similar to those of 

 Biitschli with reference to the development of the Pteropod egg 

 (8), has recently given us a very interesting account of what 

 takes place in the ripe egg of Asterias glacialis (9). In reference 

 to the formation of the polar cells, his results accord closely 

 with those of Hertwig, but he differs considerably from this 

 author with reference to the preceding changes in the germinal 

 vesicle. He believes that the germinal spot atrophies more or 

 less completely, but that in any case its constituents remain 

 behind in the egg, though he will not definitely assert that it 

 takes no share in the formation of the spindle at the expense of 

 which both the polar cells and the female pronucleus are formed. 

 The spindle with its terminal suns arises, according to him, from 

 the contents of the germinal vesicle, loses its spindle character, 

 travels to the surface, and reacquiring a spindle character is con- 

 cerned in the formation of the polar cells in the way described 

 by Hertwig. 



Giard (10) gives a somewhat different account of the be- 

 haviour of the germinal vesicle in Psammechinus miliaris. At 

 maturity the contents of the germinal vesicle and spot mix 



