8 4 



GENERAL PRINCIPLES OF ZOOLOGY. 



taining pole, in the course of development, always becomes 

 the animal pole, there can be distinguished in the egg an 

 animal part rich in protoplasm and a vegetal part rich in 

 yolk (Fig. 95). In many telolecithal eggs the two regions 

 pass gradually into one another, but in others the distinc- 

 tion is sharply expressed, so that a distinct boundary sepa- 

 rates an almost purely protoplasmic animal portion from 

 a yolk-containing vegetal portion. This condition is 

 most beautifully shown in the bird's egg (Fig. 96). Here 

 only the yellow of the egg is to be regarded as an egg in 

 the embryological sense, while the white portion, the 



7. 



FIG. 96. Diagrammatic longitudinal section through a bird's egg. (After Balfour ) ( : ) The 

 egg: b. /, blastoderm ; w.y,, white yolk ; y y., yellow yolk (2) Coverings of the egg: 

 i'.t.. yolk membrane (vitelline membrane) ; x. and ?w., inner and outer layers of white ; 

 ch.l,, chalazae ; i.s.m. and s. ;., inner and outer shell-membrane ; between them at the 

 right end is the air-chamber (a.c.h. ) \ s, shell. 



fibrillated egg-membrane, and the calcareous shell are 

 only later depositions upon the surface of the egg. The 

 chief mass of the yellow is food-yolk, upon which rests a 

 thin layer of protoplasm, the blastoderm, always uppermost 

 whatever the position of the egg. The protoplasmic layer 

 contains the egg-nucleus, and, after fertilization, by progres- 

 sive development continually separates itself more and 

 more sharply from the underlying yolk. 



Various Modes of Cleavage. After the foregoing 



