544 MATURATION AND IMPREGNATION OF THE OVUM. 



spermatozoon. The head continues to make its way by means 

 of rapid oscillations, till it has traversed about one eighth of the 

 diameter of the egg, and then suddenly becomes still. The tail in 

 the meantime vanishes, while the neck swells up and forms the 

 male pronucleus. The junction of the male and female pronu- 

 cleus is described by Fol and Selenka in nearly the same manner. 



Giard gives an account of impregnation which is not easily 

 brought into harmony with that of the other investigators. His 

 observations were made on Psammechinus miliaris. At one 

 point is situated a polar body and usually at the pole opposite to 

 it a corresponding prominence. The spermatozoa on gaining 

 access to the egg attach themselves to it and give it a rotatory 

 movement, but according to Giard none of them penetrate the 

 vitelline membrane which, though formed at an earlier period, 

 now retires from the surface of the egg. 



Giard believes that the prominence opposite the polar cells 

 serves for the entrance of the spermatic material, which probably 

 passes in by a process of diffusion. Thus, though he regards 

 the male pronucleus as a product of impregnation, he does not 

 believe it to be the head of a spermatozoon. 



Both Hertwig and Fol have made observations on the result 

 of the entrance into the egg of several spermatozoa. Fol finds 

 that when the impregnation has been too long delayed the 

 vitelline membrane is formed with comparative slowness and 

 several spermatozoa are thus enabled to penetrate. Each sper- 

 matozoon forms a separate pronucleus with a surrounding sun ; 

 and several male pronuclei usually fuse with the female pro- 

 nucleus. Each male pronucleus appears to exercise a repulsive 

 influence on other male pronuclei, but to be attracted by the 

 female pronucleus. When there are several male pronuclei the 

 segmentation is irregular and the resulting larva a monstrosity. 

 These statements of Fol and Hertwig are at first sight in con- 

 tradiction with the more recent results of Selenka. In Toxo- 

 pneustes variegatus Selenka finds that though impregnation is 

 usually effected by a single spermatozoon yet that several may 

 be concerned in the act. The development continues, however, 

 to be normal if three or even four spermatozoa enter the egg 

 almost simultaneously. Under such circumstances each sperma- 

 tozoon forms a separate pronucleus and sun. 



