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[369]. The spermatic cells originate from cells (in the case of Vertebrates 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.
Hertwig's discovery is in no way opposed to Mr Darwin's theory of pangenesis and other similar theories, but does not afford any definite proof of their accuracy, nor does it in the meantime supply any explanation of the origin of two sexes or of the reasons for an embryo becoming male or female.
Summary.
In what may probably be regarded as a normal case the following series of events accompanies the maturation and impregnation of an egg:—
(1) Transportation of the germinal vesicle to the surface of the egg.
(2) Absorption of the membrane of the germinal vesicle and metamorphosis of the germinal spot.
(3) Assumption of a spindle character by the remains of germinal vesicle, these remains being probably largely formed from the germinal spot.
(4) Entrance of one end of the spindle into a protoplasmic prominence at the surface of the egg.