THE EARLY DEVELOPMENT OF THE CHICK 239 



marks the animal pole of the ovum, is toward the attached 

 surface. 



The formation and accumulation of yolk now become more 

 rapid. The surface of the ovum is in the form of a zona radiata, 

 through the pores of which nutritive substances diffuse from 



jr.* 



FIG. 88. Section through the pigeon's ovarian ovum, 1.44X1.25 mm. From 

 Lillie (Development of the Chick), /.s., Stalk of follicle; G.V., germinal vesicle 

 or nucleus; Gr., granulosa cells; I/., latebra; p.P., peripheral protoplasm; pr.f., 

 primordial follicles; Th.ex., theca externa; Th. int., theca internal Y.Y., yellow 

 yolk; Z.r., zona radiata. 



the follicle cells, which thus function as nurse cells (Fig. 88). 

 The rate of growth of the egg during its final stages may be 

 determined by the arrangement of the concentric layers of 

 white and yellow yolk (Fig. 85, B), which mark daily periodici- 

 ties in the formation of deutoplasm (Riddle) . Toward the close 

 of the growth period the follicle becomes more clearly mem- 

 branous, and along the side opposite its attachment, which is 



