OVUM AND SPERMATOZOON 23 



mixis, is of such wide occurrence as to be almost synonymous 

 with reproduction itself, is sufficient proof of its importance. 

 Amphimixis consists in the mingling of the two parental elements 

 in reproduction ; and as we have seen that the elements necessary 

 to generation are contained in the nuclei of the ovum and sper- 

 matozoon respectively, amphimixis is a phenomenon of nuclear 

 blending. 



As conducive to amphimixis, we must first of all notice the 

 differentiation of the reproductive cells into male and female. 

 Such differentiation does not seem to affect the fundamental 

 organisation either of the egg or the sperm cell, as is borne out 

 by Boveri's experiment already mentioned. Sexual differentia- 

 tion would appear to be merely an adaptation which facilitates 

 the mingUng of the germinal substance of the two reproductive 

 cells. This mingling is brought about by the chemiotrophic 

 attraction exerted by the egg-cell. The structure of the two 

 cells bears out this view. The spermatozoon is small — ^im- 

 measurably smaller than the ovum — but it possesses greater 

 facility of movement, while the relative immobiUty of the ovum 

 is compensated for by the fact that it contains that cytoplasmic 

 substance essential to the maintenance of the life of both cells. 

 And while the ovum furnishes the nutritive matter for both, the 

 spermatozoon, by introducing a new centrosome into the egg, 

 enables the latter to develop by fission ; for the centrosome of the 

 ovum, in the majority of species, disappears more or less com- 

 pletely in the course of the egg's maturation. Thus both kinds 

 of germ-cells are essential, and each completes the other, the egg 

 famishing the nutritive capital, and the spermatozoon the 

 dynamic centre of the karyokinetic process.^ 



The conception which formerly prevailed concerning amphi- 

 mixis, and which certain experiments of Maupas on Infusorians 

 seemed to confirm, was that amphimixis afforded a sort of " re- 

 viving force " by means of which life was perpetually rejuve- 

 ^ Boveri, Das Problem der Befruchtung, pp. 32 ff. Jena, 1902. 



