GENERAL EMBRYOLOGY 



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nates (telolecitlml 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 



et.Z. 



FIG. 102. Diagrammatic longitudinal section through a bird's egg (after 

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

 Coverings of the egg: v.t., yolk membrane (vitelline membrane); x and ic., inner and 

 outer layers of white; ch.l., chalazar, i.s.m. and s.ni., inner and outer shell-membrane; 

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



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 fertilization, 

 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. 101). 



