THE REPRODUCTIVE ORGANS 



347 



the anlage of the gonad was segmental in character and 'gonotomes,' 

 comparable to nephrotomes and myotomes, were described. It has 

 since been shown that no metamerism exists and that the primary- 

 germ cells, which alone characterize the gonads, arise in several 

 groups of vertebrates (probably in all) from the entoderm, which is 

 never metameric. At about the time of the differentiation of the 

 somites they migrate through the developing mesoderm to their 

 definitive position in the epithelium of the genital ridges, the primitive 

 or primordial ova (whether to form eggs or sperm) being recognizable 

 from their size and their reaction to microscopic stains (figs. 3 7 1 , o ; 3 74) . 

 In the adults of many vertebrates the gonads at maturity 

 project far into the coelom and are often suspended by a fold of 





Fig. 371. — Section of genital ridge of chick of five days' incubation, after Semon. 

 epithelium of ridge (ccelomic wall); c, medullary cords; 0, primordial ova. 



peritoneum which is called a mesorchiimi in the male, a mesoariimi 

 in the female. 



The gonads are primitively paired ; the unpaired condition which 

 occurs in cyclostomes, many fishes and birds, is either the result of 

 fusion or of one-sided degeneration. Thus in the Ammocoete stage of 

 the lampreys the mesentery is lost and the gonads of the two sides 

 fuse. In the myxinoids, on the other hand, only the left gonad 

 normally reaches a functional stage. 



Ovaries. — In the ovarian epithelium the primtive ova multiply, 

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

 into the deeper stroma of connective tissue, thus forming ovarial 

 or medullary cords (fig. 371, c), each containing a number of ova. 

 Then the cords break up and each egg becomes surrounded with a 



