THE EGG 



27 



fully ripe follicle and is completed after ovulation in the oviduct, 

 while the ovum is being fertilized. 



The origin of the primitive ova, their multiplication and 

 the formation of the primordial follicles is described in Chapter 

 XIIL In the young chick all the cell cords and cell nests (de- 

 scribed in Chapter XIII) become converted into primordial 

 follicles. During the egg-laying period there is a continuous 

 process of growth and ripening of the primordial follicles, which 

 takes place successively; the immense majority at any given 

 period remain latent, so that at any time all stages of growth 

 of egg follicles may be found in a laying hen. 



A primordial follicle consists of the ovum surrounded by a 

 single layer of cubical epithelial cells (granulosa or follicle cells); 

 the fibers of the adjacent stroma have a concentric arrangement 

 around the follicle forming the theca folliculi (Fig. 6 8tr.). The 

 ovum itself is a rounded cell with 

 a large nucleus which may be 

 central in position or slightly ex- 

 centric. In the protoplasm on 

 one side of the nucleus is a con- 

 centrated mass of protoplasm 

 from which rays extend out into 

 the protoplasm. This is the so- 

 called yolk-nucleus; it probably 

 corresponds morphologically to 

 the attraction sphere of other 

 cells. 



Fig. 6. — Primordial follicle from the 

 ovary of the hen. (After Holl.) 



Or., Granulosa. N., Nucleus. Str., 

 Stroma. Y. N., Yolk nucleus. 



Holl derives the follicular cells 

 in birds from the stroma, but on 

 insufficient grounds. The most re- 

 cent and, in many respects, the best account is that of D 'Hollander. 

 According to this author they are derived, like the primitive ova, from 

 the germinal epithelium, in which he agrees with the majority of his 

 predecessors. He states that the period of multiplication of the ovo- 

 gonia ends about the time of hatching ; that the period of growth of the 

 ovocytes begins at about the fourteenth day of incubation (seven days 

 before hatching), and before the formation of the primordial follicle, 

 which begins on the fourth day after hatching. Thus the periods of 

 multiplication and growth overlap. He gives a detailed and well-illus- 

 trated account of the nuclear changes accompanying the first stages of 

 growth (synapsis, etc.) 



