320 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



Ovaries. In the ovarian epithelium the primitive ova multiply, 

 and the products, accompanied by some of the epithelial cells, sink 

 into the deeper stroma of connective tissue, thus forming ovarial cords 

 each containing a number of ova. Then the cords break up and each 

 egg becomes surrounded with a layer of epithelial cells, the whole 

 forming a Graafian follicle, the follicle cells supplying nourishment 

 to the contained ovum. In the higher vertebrates there is a great 

 increase in the number of follicle cells, which become arranged in 

 several layers. Then a split arises in the follicle, the cavity becoming 



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FIG. 324. Section of genital ridge of chick of five days' incubation, after Semon. e, epithel- 

 ium of ridge (ccelomic wall) ; c, medullary cords; o, primordial ova. 



filled with a follicular liquor, while the ovum, surrounded by several 

 layers of cells, adheres to one side of the cavity, this part being called 

 the discus proligerus. 



When the eggs have attained their full size and the proper time 

 has arrived some of the follicles migrate to the surface of the ovary, 

 then the follicles rupture and the contained ova escape into the ccelom. 

 Their history from this point will be outlined in connection with the 

 genital ducts. Each ruptured follicle (at least in elasmobranchs, 

 amphibians and amniotes leaves a scar on the surface of the ovary 

 the corpus luteum characterized by the presence of peculiar (' lutein ') 

 cells. 



Testes. In the gonads of the male (testes) there is a somewhat 

 similar insinking of the primordial ova and epithelial cells into the 

 stroma of the genital ridge, but, instead of breaking up into separate 

 follicles, each sexual cord develops a lumen and becomes converted 



