HORMONES IN DIFFERENTIATION OF SEX 



111 



cal course in all individuals, and a later 

 phase, the period of sexual differentiation, 

 which is chiefly hormone conditioned. Dur- 

 ing the first phase the primordial structures 

 necessary for the development of both sexes 

 are laid down and develop in similar or 

 identical fashion up to a certain point, at 

 which stage each embryo possesses, morpho- 

 logically and for a certain time, the capacity 

 to develop into an individual of either sex 

 (Fig. 2.22A). In this early, indifferent phase 

 of development the sex primordia show lit- 

 tle reactivity to hormones, capacity for re- 

 sponse evidently requiring a certain degree 

 of maturation in the reacting organs or tis- 

 sues. The onset of sexual differentiation of 

 the accessory structures follows the appear- 

 ance of sexual differentiation in the gonads, 



and this is the phase of development in 

 which control by hormones is predicated. 



A. DIFFERENTIATION OF THE 

 EMBRYONIC GONADUCTS 



A complete account of the origin and nor- 

 mal development of the embryonic sex ducts 

 has been given by Willier (1939) and Burns 

 (1955b) . In the embryos of most vertebrates 

 both sex ducts are present and equally de- 

 veloped throughout the sexually undifferen- 

 tiated period. In many amphibians this 

 primitive condition is retained throughout 

 larval life or indefinitely ; the male, or Wolf- 

 fian ducts, function as nephric ducts in both 

 sexes and in females they are permanently 

 retained in this capacity. The Milllerian 

 ducts persist throughout life in the males of 



VAS DEFERENS 



MES0NEPHR05\5 



EPIDIDYMIS 



VAGINAL CANAL 



BULBAR GLAND 



1 H PHALLUS 



V i URINOGENITAL 



^'i-*^ SINUS 



|1 > BULBAR GLAND 



POUCH YOUNG - ± 10 DAYS 



FEMALE a MALE-±35DAYS 



Fig. 2.22. Early (io\ clopnicnt and sexual differentiation in the genital tracts of young 

 opossums. A. The lusrxu.il stage of development in a female embryo ±10 days of age, show- 

 ing the paired gonaducts of both sexes, the sexually indifferent stage of the urinogenital 

 sinus, and the undifferentiated genital tubercle or phallus. B. Male and female at about 35 

 days, when sexual differentiation is far advanced, showing the structures which develop from 

 the primitive sex ducts, and dimorphic development of the sinus region. The phallus shows 

 chiefly a difference in size, without marked morphologic divergence. (From R. K. Burns, 

 Survey Biol. Progr., 1, 233-266, 1949.) 



