MATURATION AND IMPREGNATION OF THE OVUM. 129 



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 

 rtsult of the entrance into the egg of several spermatozoa. 

 Fol finds that when the impregnation has been too long 

 delayed the vitelHne membrane is formed with comparative 

 slowness and several spermatozoa are thus enabled to pene- 

 trate. Each spermatozoon forms a separate pronucleus with 

 a surrounding sun ; 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 at first sight in contradiction with the more recent 

 results of Selenka. In Toxopneiisles variegatics 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 spermatozoon forms a separate pronucleus 

 and sun. 



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. We have not as 

 yet a sufficient body of observations to enable us to decide 

 whether impregnation is usually effected by a single spermato- 

 zoon, though in spite of certain conflicting evidence the balance 

 would seem to incline towards the side of a single spermato- 

 zoon.^ 



The discovery of Hertwig as to the formation of the male pro- 

 nucleus throws a flood of light upon impregnation. 



The act of impregnation is seen essentially to consist in 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 pro- 

 nucleus is the metamorphosed head of the spermatozoon which 



' The recent researches of Calberla on the impregnalion of the ovum 

 of TetromyzoH Planeii support this conclusion. 



