404 



COMPARATIVE ZOOLOGY 



tissue paper. At the larger end it separates to inclose 

 a bubble of air for the use of the chick. Next comes 

 the albumen, or " white," in spirally ar- 

 ranged layers, within which floats the 

 yolk. The yolk is prevented from mov- 

 ing toward either end of the egg by 

 two twisted cords of albumen, called 

 chalazce ; yet is allowed to rise toward 

 with inclosed cyto- one s id e , the yolk being lighter than the 



plasm; n, nucleus, J 



consisting of nuclear albumen. 1 he yolk is composed or oily 

 gra^ui^substa'nceln granules (about gio" of an inch in diam- 

 which are seen a Qfc r \ an d j s inclosed in a sac, called 



spherical nucleolus ' 



and several irregular the vitelline membrane, and disposed in 



masses of chro- . . ... . 



matin; a, attraction concentric layers, like a set of vases 



centro e somr ining * P laCed OIle wlthm the ther - That P art 



of the yolk which extends from the 

 center to a white spot (cicatricula) on the outside can not 

 be hardened, even with the most prolonged boiling. 

 The cicatricula, or embryo spot, is a thin disk of cellular 

 structure, in which the new life first appears. This was 

 originally a simple cell, but development has gone some 



FIG. 358. Longitudinal section of .Hen's Egg before incubation: , yolk, showing con- 

 centric layers; a , its semifluid center, consisting of a white granular substance 

 the whole yolk is inclosed in the vitelline membrane ; b, inner dense part of the 

 albumen; b' , outer, thinner part; c, the chalaza, or albumen, twisted by the revolu- 

 tions of the yolk ; d, double shell membrane, split at the large end to form the 

 chamber,// e, the shell; h, the white spot, or cicatricula. 



