MATURATION AND IMPREGNATION OF THE OVUM. 545 



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 1 . 



The discovery of Hertwig as to the formation of the male 

 pronucleus 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 

 pronucleus is the metamorphosed head of the spermatozoon 

 which is itself developed from the nucleus of a spermatic cell 2 . 

 The spermatic cells originate from cells (in the case of Verte- 

 brates at least) identical with the primitive ova, so that the 

 fusion which takes place is the fusion of morphologically similar 

 parts in the two sexes. 



It must not, however, be forgotten, as Strasburger has pointed 

 out, that part of the protoplasm of the generative cells of the 

 two sexes also fuse, viz. the tail of the spermatozoon with the 

 protoplasm of the egg. But there is no evidence that the former 

 is of importance for the act of impregnation. The fact that 

 impregnation mainly consists in the union of two nuclei gives 

 an importance to the nucleus which would probably not have 

 been accorded to it on other grounds. 



1 The recent researches of Calberla on the impregnation of the ovum of Petromyzon 

 Planeri support this conclusion. 



8 This seems the most probable view with reference to the nature of the head of 

 the spermatozoon, though the point is not perhaps yet definitely decided. 



