474 



Special Vertebrate Organogenesis 



tural elements establish the sex-type of the 

 gonad, irrespective of the genetic constitution 

 of the included germ cells. If the intermedi- 

 ate mesoderm of a frog embryo is trans- 

 planted orthotopically to an embryo of dif- 

 ferent sex, the germ cells of one sex are 

 combined with the structural elements of 

 the other (Fig. 180). It is the differentiation 

 of the latter that determines the sex type of 

 the gonad (Humphrey, '33). This principle 

 applies, moreover, in all cases of functional 

 sex transformation, which end in the pro- 

 duction of gametes of the opposite sex (pp. 

 475-476). Even with respect to gamete forma- 

 tion the germ cells are indifferent or bi- 

 potential. 



APPEARANCE OF SEX-SPECIFIC 



ORGANIZATION IN THE 



GONAD 



The time at which the gonad primordium 

 becomes organized with respect to its future 

 sex has been determined by testing its ca- 

 pacity for self-differentiation at successive 

 stages of development. In amphibian em- 

 bryos transplantation of the gonad-forming 

 area shows that sex-specific organization 

 exists long before the appearance of a genital 

 ridge (Humphrey, '28a), but in these experi- 

 ments mesonephros and other regional ele- 

 ments are also present in the graft. The 

 gonad-forming area of the chick blastoderm 

 (p. 471) likewise shows a degree of sex- 

 specific organization, subject, however, to the 

 same qualification. A certain number of 

 isolates produced gonad-like bodies, with 

 sex cords of male type and the structure of 

 rudimentary testes (Fig. 179). Ovaries were 

 not formed. These experiments are not strict 

 tests of the capacity of the gonad for in- 

 dependent differentiation. 



During the formation of the genital ridge, 

 analysis shows that the capacity for specific 

 differentiation appears and increases from 

 stage to stage (see Willier, '39). After a 

 distinct germinal epithelium is present, 

 transplants of the ridge yield gonads of 

 specific sex with much higher frequency 

 than before the germinal epithelium is 

 recognizable; at earlier stages (31 somites 

 or less) only gonads of indeterminate sex 

 are formed. It may be concluded that during 

 formation of the genital ridge in the chick, 

 morphogenetic changes occur which gradvi- 

 ally determine the sex-type of the gonad. In 

 the duck embryo, as in the chick, the his- 

 tologically undifferentiated gonad is strongly 

 organized with respect to sex and, in the 



female, to laterality as well. Gonads removed 

 before the beginning of morphological sex 

 differentiation, and cultivated in vitro, pro- 

 duce structurally normal testes, or right 

 and left ovaries. Such gonads are sometimes 

 sterile (Wolff and Haffen, '52). 



The organization of the gonad primordium 

 of the rat has also been analyzed by trans- 

 plantation in various ways. Mesonephric 

 bodies isolated before a genital ridge is pres- 

 ent do not produce gonads, but in the ridge 

 stage male organization is well established 

 (Torrey, '50). When sexually undifferenti- 

 ated primordia are transplanted to the adult 

 kidney (Buyse, '35) there is a striking sex 

 difference in capacity for self-differentiation. 

 (This study was carried out to test the pos- 

 sibility that the differentiation of the grafted 

 gonads might be modified by the hormones 

 of the host. From this standpoint the results 

 were negative.) Testes develop normally but 

 prospective ovaries lack stable organization 

 and give rise to the following types: (1) 

 retarded ovaries; (2) ovotestes, in which both 

 gonad components are developed but more or 

 less rudimentary; (3) rudimentary testes, 

 in which only the medullary component of 

 the ovary has survived; and (4) grafts of 

 indeterminate sex. Evidently, testis organiza- 

 tion is strongly fixed in the indifferent 

 primordium (cf. Torrey, '50) but the pro- 

 spective ovary is extremely labile, the medul- 

 lary component developing with about the 

 same frequency as the cortical. Similar 

 results were obtained after transplantation 

 to other sites (Moore and Price, '42;* Holy- 

 oke, '49), with indications that the lability of 

 the ovary is related to its age at transplanta- 

 tion. 



Further analysis of the age factor (Torrey, 

 '50) shows that the organization of the 

 testis is fixed as early as the 12-day pri- 

 mordium (early gonadal blastema stage). 

 The critical period for the ovary comes much 

 later, at 15 to 16 days (cf. Moore and Price), 

 when grafts may give rise to ovaries, ovo- 

 testes or rudimentary testes. By the seven- 

 teenth day (when a cortex is histologically 

 present) all grafts produce typical ovaries. 

 Evidently the sex-specific organization of the 

 gonad primordium is gradually acquired and 

 is realized much earlier in the testis than in 

 the ovary. This fact corresponds with the 

 normal order of development, in which the 

 medullary component (primary sex cords) 

 has priority of origin in both sexes. It is in 



* These authors did not describe distinct ovotestes 

 but "transformed" and hypertrophied medullary- 

 cords were frequently found. 



