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THE MESODERMAL DERIVATIVES 



Posterior veno cava 



3lood corpuscles 



Mesonephric 



(Wolffian) 



duct 



Germinal epithelium 

 Primordial germ cell 

 Coelom 



Gonad onlage 



Dorsal mesentery (splonchnopleure) 



— Presumed path of primordial germ 

 cells to gonad anioge 



Gonad primordia of the 1 1 mm. frog tadpole. 



This double membrane is known as the mesorchium in the male and 

 the mesovarium in the female. 



These coelomic projections of pre-gonad material are known as 

 genital ridges. Sex is, of course, not distinguishable as the gonad 

 primordium shows no particular cellular differentiation at this time. 

 The general organization of both presumptive ovary and testis is one 

 of an enlarged sac with germ cell primordia around the periphery 

 and the primary genital cavity in the center. 



The rete cords now appear as strands of cells which arise from 

 the developing Malpighian bodies of the mesonephros and grow into 

 the primary genital cavity of the undifferentiated gonad. These 

 strands unfortunately are referred to in some textbooks as sexual 

 cords or sex cords, but these terms are confused too easily with sex 

 cell ridge or sex cell cords, terms just used. They properly are called 

 rete cords. 



Sex differentiation occurs before the time of metamorphosis, at 

 about the 30 mm. body length stage. The gonads show macroscopic 

 differences at this time. In the case of the potential ovary the cells 

 around the periphery of the gonad primordium multiply and the rete 

 cord strands, which have invaded the genital cavity, begin to develop 

 their own cavities, which are known as the secondary genital cavities. 

 These become confluent and form from seven to twelve large ovarian 

 sacs, each connected with the others. The cellular material of the 

 rete cords then gives rise to clusters of cells, one of which begins 

 to grow toward the primary oocyte stage. The others (primordial 

 germ cells) become the surrounding nurse or follicle cells. All these 



