FERTILISATION OF THE EGG. 15 



sister threads. A spindle figure, with pole bodies and polar rays 

 at its apices now appears, and the outlines of the pronuclei, pre- 

 viously distinct, disappear. The chromatin loops, of which, 

 owing to the longitudinal splitting, there are now eight, four 

 male and four female, take up a position at the equator of the 

 spindle. The two sister threads of each pair then separate, one 

 moving towards one pole of the spindle, the other towards the 

 opposite pole ; so that at each pole of the spindle there is a group 

 consisting of two male threads and two female threads. Each 

 group then forms a daughter nucleus, by fusion of the threads to 

 form a skein, and the entire egg divides into the two first seg- 

 mentation cells, or blastomeres. 



This equal division of male and female elements in the first 

 segmentation is of great interest in reference to the theory of 

 fertilisation. If it should prove to occur in the later, as well as 

 in the earliest stages of development, then, as pointed out above, 

 it will follow that the nucleus of each cell in the body of an adult 

 animal will contain male and female elements derived from the 

 male and female pronuclei, i.e. from the father and the mother, in 

 precisely equal amounts. In other words, each cell of the adult 

 body may be spoken of as hermaphrodite. If this be true, then 

 the egg, which in its first origin is merely an epithelial cell, 

 must itself be hermaphrodite. 



Theory of Fertilisation. 



The view developed above, that the egg is to be regarded as 

 hermaphrodite, led to the suggestion, by Minot, that the extrusion 

 of polar bodies may be an act by which the egg gets rid of its 

 male elements ; a view adopted by Balfour, who added the further 

 suggestion that after the formation of the polar bodies the part 

 of the egg nucleus remaining within the ovum, i.e. the female 

 pronucleus, is incapable of further development without the 

 addition of the nuclear part' of the male element, or male pro- 

 nucleus ; and that the habit of forming polar bodies has been 

 acquired by ova for the express purpose of preventing partheno- 

 genesis, and of rendering fertilisation indispensable. 



This view is exceedingly suggestive, and with slight modifi- 

 cations has been widely adopted. There are, however, very 

 serious objections to it. It does not explain why the formation 

 of polar bodies so closely resembles, or is even identical with. 



