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, 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 Str.). The 
ovum itself is a rounded ceil 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- 
‘alled yolk-nucleus; it probably 
corresponds morphologically to 
the attraction sphere of other 
cells. FG. 6.— Primordial follicle from the 
Holl derives the follicular cells Creer pe ae (After Hol) 
: ; ; Gr., Granulosa. N., Nucleus. Str. 
in birds from the stroma, but on stroma. Y. N., Yolk nucleus. : 
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.) 
