HORMONES IN DIFFERENTIATION OF SEX 



139 



inhibits the male type of development (pp. 

 127, 131) which, on the other hand, develops 

 spontaneously in both sexes after castration 

 (Fig. 2.25) or after isolation in vitro, with- 

 out hormonal conditioning. A difference in 

 susceptibility to inhibition by the female 

 hormone apparently masks the inherent dif- 

 ference in growth potential and the primary 

 symmetry of the female structure is pre- 

 served. 



The marked asymmetries of the genital 

 system in birds thus appear to rest on lat- 

 eral differences involving such physiologic 

 characteristics as growth potentials and 

 reaction thresholds. These in turn are ap- 

 parently correlated with a more extensive 

 asymmetry involving the whole organism. 

 Lateral growth differentials are established 

 in the blastoderm of chick embryos as early 

 as the head-process stage. In testing the 

 organ- forming potencies of regional pieces 

 of the blastoderm it was found (Willier and 

 Rawles, 1935; Rawles, 1936) that when cor- 

 responding pieces of the same size from right 

 and left halves of the blastoderm are trans- 

 planted to the chorioallantoic membrane 

 they consistently show marked differences 

 in capacity for growth and self-differentia- 

 tion, which are manifested both in the size 

 attained by the graft (growth capacity) and 

 in the quality of the histologic differentia- 

 tion. Regardless of the particular tissues or 

 organs dealt with, grafts from the left half 

 of the blastoderm are consistently larger 

 and better differentiated than those from the 

 corresponding pieces of the right half. Lillie 

 (1931) also postulated critical differences 

 in growth rate and threshold to sex hor- 

 mones on the two sides of the body in 

 explaining the occurrence of "gynandromor- 

 phic" plumage in adult fowls and its dis- 

 tribution. He pointed out that the sharply 

 defined difference in plumage on the two 

 sides of the body is usually accompanied in 

 gynandromorphs by gross bodily asym- 

 metry (hemihypertrophy) favoring the left 

 side. It would seem that lateral differences 

 in the morphologic and physiologic proper- 

 ties of the sex primordia of birds are not 

 peculiarities of sexual differentiation ; rather 

 they are an aspect of the general pattern of 



somatic organization in this group. In the 

 normal differentiation of sex as well as in 

 experimental studies these differences are 

 exploited by hormones whose effects serve 

 merely to exaggerate or to obliterate tend- 

 encies inherent in the organization of the 

 individual primordia. 



C. INFLUENCE OF SEX GENOTYPE ON THE 

 REACTIONS OF SEX PRIMORDIA 



Another example of the way in which 

 constitutional factors operate to modify or 

 set limits to hormone action is seen in the 

 influence of the sex genotype on the re- 

 sponses of sex primordia to hormones, as il- 

 lustrated especially in young opossums 

 (Burns, 1942b, 1956a). In comparing the 

 effects of identical doses of the same hor- 

 mone (whether male or female) in embryos 

 of different sex, it was found in the case of 

 many structures that the amount of growth 

 induced was influenced by the sex of the 

 individual. Under identical experimental 

 conditions the effect of a male hormone on 

 the growth of a particular male structure 

 was always greater in male embryos than on 

 the homologous structure in females, and 

 vice versa. This result is well illustrated by 

 the reactions of the genital tubercle or phal- 

 lus in male and female littermates which 

 received identical doses of testosterone pro- 

 pionate. The transformed phallus of the fe- 

 male cannot be distinguished anatomically 

 or histologically from the male organ, ex- 

 cept for a constant and considerable dif- 

 ference in size (Fig. 2.32). Such an effect 

 was cited earlier in the case of the prostate 

 (Fig. 2.29) and it occurs for various other 

 male structures such as the vas deferens 

 I Wolffian duct) and the epididymis (Fig. 

 2.24). After treatment with female hormone 

 corresponding differences are observed in the 

 response of female structures. The Miiller- 

 ian ducts of male embryos hypertrophy and 

 undergo a typical differentiation into ovi- 

 duct and uterus; nevertheless, in size these 

 organs do not approach those produced by 

 the same dosage in females (Fig. 2.24). The 

 same is true in the case of the hyperplastic 

 reaction of the sinus epithelium and for 

 other structures. Differences in size can be 



