GENERAL EMBRYOLOGY. 



153 



whatever the position of the egg. The protoplasmic layer contains 

 the egg-nucleus, and, after fertilization, by progressive develop- 



oh.l 



FIG. 99. Diagrammatic longitudinal section through a bird's egg. (After Balfour.) 

 (1) The egg: b.J., blastoderm; w.y., white yolk: y y., yellow yolk. (2) Coverings of 

 the egg: v.t., yolk membrane (vitelline membrane); x. and w., inner and outer 

 layers of white; cto.L, chalazae; i.s.m. and a.m., inner and outer shell-membrane; 

 between them at the right end is the air-chamber (a.c./L); s, shell. 



ment continually separates itself (blastoderm) more and more 

 sharply from the underlying yolk. 



Various Types of Cleavage. After the foregoing remarks a 

 brief explanation will suffice to render intelligible the following 

 figures of the various modes of cleavage. 



a. Holoblastic Eggs with Total Cleavage. 



1. Equal Cleavage. The yolk, present only in small quantity, 

 is distributed equally through the egg; upon cleaving, the egg 

 divides into parts of approximately the same size and equally rich 

 in yolk (alecithal eggs, fig. 96). 



2. Unequal Cleavage. The yolk is abundant, but not in such 

 a quantity as to prevent complete cleavage; it lies especially at the 

 vegetative pole of the egg, causing the cleavage in this region to 

 progress more slowly; here larger cleavage spheres are formed, 

 because richer in yolk; hence the embryo, at the very first, is 

 found to be composed of smaller animal cells poor in yolk, and 

 larger vegetative cells rich in yolk (telolecithal, holoblastic eggs, 

 figs. 100 and 101). In some instances of unequal cleavage the 



