MATURATION AND IMPREGNATION OF THE OVUM. 115 



pronucleus — and travelling towards the centre of the egg under- 

 goes a fate which will be spoken of in the second part of this paper. 



The most obscure part of HerLwig'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 development 

 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 Biltschli 

 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 Asierias glacialis (9). In reference to the for- 

 mation 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 e^^, 

 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 concerned in the 

 formation of the polar cells in the way described by Hertwig. 



Giard (10) gives a somewhat different account of the behaviour 

 of the germinal vesicle in Fsammechhms miliaris. At maturity 

 the contents of the germinal vesicle and spot mix together and 

 form an amoeboid mass, which, assuming a spindle form, divides 

 into two parts, one of which travels towards the centre of the 

 egg and forms the female pronucleus, the other remains at the 

 surface and gives origin to two polar cells, both of which are 

 formed after the egg is laid. What Giard regards as the female 

 pronucleus is perhaps the lower of the two bodies which take 

 the place of the original germinal vesicle as described by Tol. 

 Vide the account of Fol's observations on p. 117. 



Strasburger, from observations on Phallusla, accepts in the 

 main Hertwig's conclusion with reference to the formation of 

 the polar bodies, but does not share Hertwig's view that either 

 the polar bodies or female pronucleus are formed at the expense 

 of the germinal spot alone. He has further shown that the so- 

 called canal-cell of conifers is formed in the same manner as the 



