THE MESOMERE (INTERMEDIATE CELL MASS) 229 



A. 



Primordial germ cells (g.c.) in the tadpole of the common frog {Rana 

 temporaria). (A) In the mesentery (m.). (B) In the genital ridge; (g.ep.) 

 germinal epithelium; (/.c.) follicle-cell. (Courtesy, Jenkinson: "Vertebrate 

 Embryology," Oxford, The Clarendon Press.) 



cells probably are derived from the outer germinal epithelium. They 

 are found at all times as clusters of potential ovarian follicles, both 

 nurse cells and ova. Many primordial germ cells are expelled from 

 the forming follicles and disintegrate within the peritoneal cavity. 



In the testes the rete cord material remains condensed to give 

 rise to germ cells which migrate from the periphery to the gonad 

 primordium, to form cysts. These cysts give rise to the seminiferous 

 tubules of the newly metamorphosed male frog in which all stages 

 of spermatogenesis will develop. Each seminiferous tubule is con- 

 nected with a collecting tubule and vasa efferentia, all within the rete 

 cord material. Some of the rete cord cells give rise to the interstitial 

 (endocrine) and connective (stroma) tissue of the testis. 



Fat bodies develop from one-third to one-half of the anterior ends 

 of either of the genital ridges. These are storage masses built up dur- 

 ing the summer feeding periods in anticipation of hibernation and the 

 subsequent active spring breeding period. In closely related toads of 

 certain species the anterior end of the testis often may develop a 

 rudimentary ovary, with oocytes, which will respond to hormonal 

 treatment much in the manner of a normal ovary. This is known as 

 Bidder's organ. Isolated oocytes have been found in the seminiferous 



