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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Fic. 324.—Section of genital ridge of chick of five days’ incubation, after Semon. ¢, epithel- 
ium of ridge (coelomic wall); c, medullary cords; 0, 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 
