THE EARLY DEVELOPMENT OF THE CHICK 237 



brief period of nearly or quite complete cessation. This is 

 quite in contrast to the great majority of animals, in which a 

 large number of ova are produced within a very brief period, 

 a condition probably correlated with the very large size of the 

 ova, for the formation of even a single egg requires a consider- 

 able expenditure of energy and substance. Moreover, there is 

 space in the organs of reproduction for but a very few ova of 

 such large dimensions. Such a succession in the formation of 

 the ova makes it possible to observe, in a single ovary, most of 

 the steps in the formation of the fully grown ovum. 



The first phase of oogenesis, the multiplication of the oogonia, 

 occurs during the embryonic life of the chick, and is practically 

 completed by the time of hatching. All of the ova produced 

 later, during the period of adult life, are thus in the form of 

 primary oocytes at the "birth" of the chick. Surrounded by 

 the non-germinal cells of the "germinal" epithelium, the oogo- 

 nia or primitive ova multiply rapidly. Some of them leave the 

 epithelium and migrate into the stroma of the ovary where they 

 degenerate. The remaining oogonia, which commence to 

 enlarge while still continuing their multiplication, together with 

 the rapidly proliferating epithelial cells, then form elongated 

 strands or cords, extending from the epithelium into the stroma. 

 Soon these definitive oogonia cease multiplication and are then 

 to be termed primary oocytes (Fig. 87, A). The strands then 

 break up into cell groups or "nests," each consisting of a single 

 primary oocyte surrounded by a number of the original epithe- 

 lial cells; these latter take up a definite epithelial arrangement 

 around the oocyte, and thus form the primitive egg follicle 

 (granulosa cells). This arrangement of the cells occurs a few 

 days after hatching. The structure of the oocyte follicle at 

 this age is shown in Fig. 87, B. 



The egg cell, both nucleus and cytoplasm, now begins to 

 enlarge and deutoplasmic granules are laid down all around the 

 centrally located nucleus, and throughout the cytoplasm, except 

 in its peripheral region which remains comparatively free from 

 yolk. This peripheral protoplasmic layer is definitely thickened 

 at one point, namely, toward the attached surface of the ovum 



