442 



EMBRYOLOGY OF THE CHICK 



OOGENESIS 



The process by which the eggs already present in the ovary of the 

 new-born chick originally came to be what they are is known as oogene- 

 sis. The first event in oogenesis is known as the multiplication of the 

 oogonia. This occurs during the embryonic period of the animal. There 

 are two types of cells which develop from the original primary cells 

 set aside for reproductive purposes. How and why these differentiate 

 in the way they do we do not know, but we do know that there is a dif- 

 ferentiation. 



As soon as these original cells commence to divide, some of them 

 develop into centrally located eggs or ova (Fig. 254), while others, 

 known as germinal epithelium, surround the more centrally located ova 

 and form a sort of case or capsule around them. The primitive egg 

 surrounded by this epithelial case is known as an oogonium. Some of 

 these leave the epithelium and pass into the stroma of the ovary. There 

 they degenerate. Those remaining, however, begin enlarging even while 

 they are dividing and multiplying. The epithelial cells also divide very 

 rapidly, forming long strands or cords which in turn extend into the 



stroma. There comes a 

 time when these primitive 

 ova or oogonia stop multi- 

 plying; they are then called 

 primary oocytes. At this 

 time the strands or cords of 

 germinal epithelium break 

 up into little groups some- 

 times called nests. Each 

 nest consists of a single pri- 

 mary oocyte (Fig. 255) sur- 

 rounded by a number of the 

 original epithelial cells. 

 These latter cells form a 

 definite case surrounding the 

 oocyte. The case thus 

 formed is called the primi- 

 tive egg follicle. This final 

 arrangement takes place 

 within a few days after 

 hatching. It will thus be 

 seen that all the eggs which 

 enlarge, ripen, and pass out of the ovary are merely enlarged and devel- 

 oped primary oocytes. 



Both the nucleus and the cytoplasm of the egg cell now begin to 

 enlarge, and yolk granules are laid down all about the centrally located 



86. 



Section of the Germinal Epithelium and Adjacent Stroma 



in a Chick-Embryo. 



g.ep., germinal epithelium forming a thickened 

 ridge-like projection ; pr.ov., primitive ova of various 

 sizes, some in the germinal epithelium and others some- 

 what beyond the limit of this epithelium ; st., strands of 

 cells which have grown from the germinal epithelium, 

 and one of which appears connected with an enlarged 

 primitive ovum. (From Semon.) 



