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 

 XIII. 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, but 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 folhculi (Fig. 6 Str.). The 

 ovum itself is a rounded cell with 

 a large nucleus placed excentri- 

 cally so as to define a primary 

 axis of the ovum. In the pro- 

 toplasm on one side of the 

 nucleus is a concentrated mass 

 of protoplasm, the yolk-nucleus, 

 from w^hich rays extend, and 

 minute fatty granules. 



HoU derives the follicular cells 

 in birds from the stroma, but on 

 insufficient grounds. According to 

 D'Hollander, they are derived, like 

 the primitive ova, from the germi- 

 nal epithelium, in which he agrees 

 with the majority of his predeces- 

 sors. He states that the period of 

 multiplication of the ovogonia ends about the time of hatching; that the 

 period of growth of the ovocytes begins at about the fourteenth da}- of in- 

 cubation (seven days before hatching), and before the formation of the 

 primordial follicle, which begins on the fourth day after hatching. Thus 

 the periods of multipHcation and gro\\i^h overlap. 



Although the nucleus (germinal vesicle of authors) is strongly 

 excentric in position in the youngest ovocytes, it occupies a more 

 nearly central position in those slightly older. When the ovum is 

 about 0.66 mm. in diameter, it moves to the surface along the 

 shortest radius and comes to lie almost in contact with the vitel- 



FiG. 6. — Primordial follicle from the 

 ovary of the hen. (After Holl.) 



Gr., Granulosa. N., Nucleus. Str., 

 Stroma. Y. N., Yolk nucleus. 



