346 



ANIMAL DEVELOPMENT 



them, to be employed in building up the body of the embryo. 

 The quantity and distribution of this material have much to do 

 with the nature of the cleavage which succeeds fertilization. 

 Unless a great deal of it is present the cleavage is total, the whole 

 of the egg-cell taking part in the divisions which produce the blastula. 

 But it happens in many cases that the yolk is not uniformly distri- 

 buted, being concentrated towards one end (or "pole"), and the 



Fig. 865. Influence of Food-yolk on Development. Diagrammatic sections 



Read each column from above downwards. The food-yolk represented by dots. The two upper lines 

 skow early stages in division of the ovum, and the bottom line the blastula stage. A, Total equal cleavage 

 (no yolk). B and c, Total unequal cleavage (yolk at one pole). D, Partial cleavage (central yolk). E, Partial 

 cleavage (yolk at one pole). For convenience' sake all the figures are made of the same size, but the size of an 

 ovum is proportionate to the amount of food-yolk it contains, e.g. A would be very small and E very large. 



cleavage-cells are larger in this region. From this we pass to 

 egg-cells which are so crammed with yolk that cleavage is partial, 

 i.e. only that part of the cell which is not thus encumbered divides. 

 This may be a patch at one pole, as in the eggs of Dog- Fishes and 

 Birds, or it may be a surface layer, as in Arthropods, where the 

 yolk is massed in the centre. 



In egg-laying or oviparous animals, as opposed to viviparous 

 forms, in which the young animal is brought into the world more 

 or less ready to commence independent existence, the time of 

 hatching depends upon the amount of yolk, or of this and other 

 nutritive matters which may be enclosed in a protective covering 



