FERTILISATION OF THE EGG. 117 



By this time the second polar body has been formed and 

 extruded, and the female pronucleus is the only part of the 

 original egg nucleus still remaining. The male and female pro- 

 nuclei, which are at first some little distance apart, rapidly 

 approach each other, come into close contact, and after having 

 increased considerably in size, fuse together, about two and a 

 half hours after fertilisation has commenced, to form the 

 segmentation nucleus. 



The segmentation nucleus is a large spherical vesicle im- 

 bedded in finely granular protoplasm, and surrounded by an 

 ill-defined capsule of pigment : its formation by the fusion of the 

 male and female pronuclei completes the act of fertilisation. 



The female pronucleus may be regarded as an imperfect 

 nucleus, and the upshot of the process of fertilisation is the 

 completion of this nucleus ; the nucleus of the spermatozoon, or 

 male pronucleus, replacing the part of the egg-nucleus which 

 has been lost as the polar bodies. The further explanation of 

 the sexual process is probably to be found in the great advan- 

 tage, as regards vigour of offspring, that is known to result 

 in both animal and vegetable kingdoms from cross fertilisation, 

 i.e., from combining the energies of two distinct individuals in 

 the act of reproduction. 



D. Segmentation of the Egg. 



The earliest stages of development consist in repeated division 

 -of the egg, whereby it becomes converted from the unicellular 

 condition, which is permanent only in the lowest animals, to the 

 multicellular state characteristic of all higher animals. To these 

 early processes of development the name Segmentation is given. 



Very shortly after the completion of the act of fertilisation 

 and formation of the segmentation nucleus this latter loses its 

 •spherical form and becomes spindle-shaped, the yoke granules at 

 the same time showing a tendency to arrange themselves in lines 

 radiating outwards from the ends of the spindle. The nucleus 

 now divides into two halves, which move away from each other; 

 the yolk granules tend to aggregate themselves around the two 

 nuclei, and a thin vertical plate of finely granular substance is 

 left, bisecting the egg. 



At the upper or black pole of the egg a depression now 

 appears, at first as a small pit and then as a groove, which 

 soon extends all round, and, rapidly deepening, divides the egg 



