IMPREGNATION OF THE OVUM. 85 



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 up to a certain point 

 in contradiction with the more recent results of Selenka. In 

 Toxopneustes variegatus Selenka finds that though impregnation 

 is usually effected by a single spermatozoon yet several may be 

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

 be normal up to the gastrula stage, at any rate, if three or even 

 four spermatozoa enter the egg almost simultaneously. Under 

 such circumstances each spermatozoon forms a separate pro- 

 nucleus and star. Selenka is of opinion (apparently rather on 

 a priori grounds than as a result of direct observation) that 

 normal development cannot occur when more than one male 

 pronucleus fuses with the female pronucleus ; and holds that, 

 where he has observed such normal development after the 

 entrance of more than one spermatozoon, the majority of male 

 pronuclei become absorbed. 



It may be noticed that, while the observations of Fol and 

 Hertwig were admittedly made upon eggs in which the impreg- 

 nation was delayed till they no longer displayed their pristine 

 activity, Selenka's were made upon quite fresh eggs ; and it 

 seems not impossible that the pathological symptoms in the 

 embryos reared by the two former authors may have been due 

 to the imperfection of the egg, and not to the entrance of more 

 than one spermatozoon. This, of course, is merely a suggestion 

 which requires to be tested by fresh observations. 



Kupffer and Benecke have further shewn that although only 

 one spermatozoon enters the ovum directly in Petromyzon yet 

 other spermatozoa pass through the vitelline membrane, and are 

 taken into a peculiar protoplasmic protuberance of the ovum 

 which appears after impregnation. 



The act of impregnation may be described as the fusion of 

 the ovum and spermatozoon, and the most important feature in 

 this act appears to be the fusion of a male and female nucleus ; 

 not only does this appear in the actual fusion of the two pro- 

 nuclei, but it is brought into still greater prominence by the fact 

 that the female pronucleus is a product of the nucleus of a 

 primitive ovum, and the male pronucleus is the metamorphosed 



