Urinogenital System 



471 



composite gonads containing both germ cells 

 and structural elements derived from the 

 graft (Nieuwkoop, '47). In tail-bud stages 

 the two elements are still closely associated 

 in the intermediate mesoderm (Humphrey, 

 '28a,b). Transplantation of the gonad-meso- 

 nephros complex at this stage shows it to be 



of the blastoderm (Fig. i72A; see Rawles, 

 '36). (At this stage the germ cells are pre- 

 sumed to be located peripherally, in the 

 "germ cell crescent"; p. 472.) Transplants 

 which include the anterior end of the primi- 

 tive streak and node produce gonad tissue 

 containing testis cords (in association with 



Primary sex cords 



Germinal epi+he 



Cor + ical cords 

 (secondary sex cords) 



nal epi+heli 



Fig. 178 Diagrams illustrating the origin of medullary and cortical components in the histologically more 

 complex gonads of mammals. A, Genital ridge, with incipient primary sex cords originating from the 

 germinal epithelium. B, Indifferent gonad, in which the medullary component is present as the primary sex 

 cords, while the cortex is potentially represented by the germinal epithelium. C, Development of the testis, 

 with disappearance of the germinal epithelium. D, Development of the ovary, with the production of 

 dominant secondary sex cords and cortex. 



capable of autonomous differentiation. Gon- 

 ads of specific sex are produced, the fre- 

 quency increasing with age. 



In the tail-bud stage of anuran embryos, 

 on the other hand, only the structural ele- 

 ments are found in the mesoderm; the germ 

 cells lie in the "yolk ridge." When the inter- 

 mediate mesoderm is exchanged between em- 

 bryos of different sex, the gonads which 

 develop always correspond to the sex of the 

 donor embryo (Humphrey, '33). Thus the 

 capacity to produce a gonad of specific sex 

 resides in the structural elements. 



In the chick a localized gonad-forming 

 area exists as early as the head-process stage 



mesonephros and adrenal cortex), but such 

 gonads are sterile (Willier, '37, '50). The 

 structural elements of the gonad are thus 

 localized topographically long before the 

 formation of a gonad primordium. 



In the embryos of mammals little is known 

 of the localization of the structural elements 

 before the stage of the genital ridge, but 

 close association with the nephrogenic ma- 

 terial is assumed. 



The Primordial Germ Cells. The history of 

 the primordial germ cells of vertebrates has 

 long been a controversial field. The classic 

 theory of early segregation, as opposed to a 

 later and more local origin, has been sup- 



