THE EARLY DEVELOPMENT OF THE CHICK 233 



area pellucida, and a whitish peripheral area opaca. The great 

 mass of the ovum or vitellus is composed of the deutoplasm or 

 "yolk," of which two forms are present, known as white and 

 yellow yolk. The white yolk occupies the region just beneath 

 the blastoderm, and extends thence as a flask-shaped mass, to 



vm 



ad 



pv 



FIG. 85. Semidiagrammatic illustration of the hen's egg at the time of laying 

 A. Entire "egg." Modified from Marshall. B. Diagram of a vertical sec- 

 tion through the vitellus or ovum proper, showing the concentric layers of white 

 and yellow yolk, o, Air chamber; ac, chalaziferous layer of albumen; ad, dense 

 layer of albumen; a/, fluid layer of albumen; b, blastoderm; c, chalaza; I, latebra; 

 nl, neck of latebra; P, nucleus of Pander; pv, perivitelline space; smi, inner 

 layer of shell membrane; smo, outer layer of shell membrane; v, vitellus or "yolk" ; 

 vm, vitelline membrane; wy, layers of white yolk; yy, layers of yellow yolk. 



the center of the whole vitellus; surrounding this the deuto- 

 plasm is arranged in several concentric layers, thick layers of the 

 yellow yolk alternating with thinner strata of white yolk (Fig. 

 85, B). These two forms of yolk differ in physical characteris- 

 tics other than color, and also in chemical composition; both are 

 made up of yolk spheres or plates, which in the white yolk are 

 smaller and quite variable in size and form. 



