GENERAL EMBRYOLOGY 



143 



nates {telolecithal eggs). Since the nuclear pole always becomes the ani- 

 mal pole, there can be distinguished in the egg an animal part rich in pro- 

 toplasm and a vegetative part rich in yolk. In many telolecithal eggs the 

 two regions pass gradually into one another, but in others a distinct 

 boundary separates an almost purely protoplasmic animal portion from a 

 yolk-containing vegetative portion. This is well shown in the bird's egg 

 (fig. 102). Here only the yolk is to be regarded as an egg in the embryo- 

 logical sense, while the white, egg-membrane, and calcareous shell are 

 depositions upon the surface of the egg. The chief mass of the yolk is 



eh.7. 



Fig. 102. — Diagrammatic longitudinal section through 

 Balfour), (i) The egg: b.l., blastoderm; w.y., white yolk; 



Coverings of the egg: v.t., yolk membrane (\'itelline membrane); .v and to., inner and 

 outer layers of \\'hite; ch.l., chalazcc; i.s.m. and s.m.^ inner and outer shell-membrane; 

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



a bird's egg (after 

 y.y., yellow yoUc. (2) 



deutoplasm, upon which rests a thin layer of protoplasm, the germinal 

 disc, always uppermost in the bird's egg, whatever the position of the egg. 

 The protoplasmic layer contains the egg-nucleus, and, after fertihzation, 

 by progressive development is separated {blastoderm) more and more 

 sharply from the underlying yolk. 



Types of Cleavage. — A brief explanation will now render the following 

 figures of the various modes of cleavage intelligible. 



a. Holoblastic Eggs with Total Cleavage. 



I. Equal Cleavage. — The yolk, present only in small quantity, is distributed 

 equally through the egg; upon cleaving, the egg divides into parts of approxi- 

 mately the same size and equally rich in yolk {alecithal eggs, fig. loi). 



