IMPREGNATION OF THE OVUM. 69 



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 sperma- 

 tozoa are thus enabled to penetrate. Each spermatozoon forms a 

 separate pronucleus with a surrounding star; and several male 

 pronuclei usually fuse with the female pronucleus. 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 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 pronucleus 

 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 prouucleus; 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 impregnation 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 obser- 

 vations. 



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 pronuclei, 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 head of the spermatozoon 

 which, as stated above, contains part of the nucleus of the primitive 

 spermatic cell. The spermatic cells originate from cells indis- 

 tinguishable from the primitive ova, so that the fusion which takes 

 place is the fusion of morphologically similar parts in the two sexes. 



