SEGMENTATION OF THE EGG 109 



explanation of the sexual process is probably to be found 

 in the great advantage, as regards vigour of offspring, 

 that is known to result in both animal and vegetable 

 kingdoms from cross fertilisation, i.e., from combining the 

 protoplasm of two distinct individuals in the act of repro- 

 duction. 



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 yolk 

 granules at the same time showing a tendency to become 

 arranged 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 into two completely separate halves along 

 a plane corresponding with the vertical plate mentioned 

 above. 



Each of the two nuclei soon divides again into two, and a 

 second cleft is formed in the same manner as before : it also 

 is vertical, but in a plane at right angles to the first one, and 

 on its completion the egg consists of four precisely similar 

 segments or cells, each with a nucleus. 



The third cleft is horizontal, but not equatorial, lying 

 nearer the upper than the lower pole : it divides each of the 

 four cells into two, an upper smaller and a lower larger 

 one. 



