140 



BIOLOGIC BASIS OF SEX 



detected at an early stage and increase 

 throughout the period of treatment. 



To account for the constancy of these dif- 

 ferences it seems necessary to assume that 

 sex constitution in some way conditions the 

 reactivity of the primordia, placing certain 

 limitations on rate of growth. When sex 

 constitution and type of hormone adminis- 

 tered are the same there is, in effect, a sum- 

 mation of the two factors, but when they 

 are different a conflict occurs. It might seem 

 more simple to suppose that the embryonic 

 gonads and their hormones are involved in 

 this result, rather than to assume a differ- 

 ential reactivity on the part of the sex pri- 

 mordia; hormones of the same type would 

 in one instance reinforce each other whereas 

 in the other case unlike hormones oppose 

 each other. However, this simple hypothe- 

 sis cannot be sustained. Although the pres- 

 ence of a hormone from the embryonic testis 

 may be safely accepted, there is as yet no 

 evidence in mammals that the ovary pro- 

 duces significant amounts of hormone at this 

 period, thus no supplementary factor can 

 be assumed in the case of females receiving 

 female hormone. In the male, moreover, his- 

 tologic studies reveal that the size differ- 

 ences observed are much too great to be ac- 

 counted for by the secretory activity of the 

 embryonic testis ; for example, the difference 

 in size between the male and the female 

 prostate after treatment with male hormone 

 (Fig. 2.29) is many times greater than the 

 volume of the normal prostate, which may 

 be taken as the measure of normal testis 

 activity. The conclusive argument against 

 participation of embryonic hormones in this 

 phenomenon has come, however, from ex- 

 amination of the embryonic testes of ex- 

 perimental animals (Burns, 1956a). There 

 is a great reduction in the size of the testis 

 (and the ovary as well) and histologic study 

 shows complete suppression of the inter- 

 stitial tissue, the intertubular spaces being 

 filled with a dense, nonstaining connective 

 tissue of mucoid type. In contrast, the nor- 

 mal testis of the same age has a rich inter- 

 stitium which is well developed as early as 

 the 10th day of pouch life (Fig. 2.17.4 ». 

 Evidence to be summarized later points 

 strongly to the embryonic interstitial tissue 

 as the source of the testis hormone, and in 



the absence of this tissue it does not seem 

 that the testis can be a factor in the result. 



IX. The Time Factor in the Responses 



of Sex Primordia: Receptivity 



and "Critical Periods" 



Of special importance is the factor of de- 

 velopmental age as it relates to the appear- 

 ance of receptivity and the timing of de- 

 terminative changes in sex primordia. This 

 becomes apparent when the reactivity of a 

 primordium to sex hormones, or its capacity 

 for independent differentiation after isola- 

 tion, is tested at successive stages of devel- 

 opment. Typical studies of the second type 

 are the experimental analyses of the appear- 

 ance of sex-specific organization in the geni- 

 tal ridge of chick embryos (Willier, 1933) 

 and in the differentiating gonads of the rat 

 (Torrey, 1950; see pp. 103, 104). Such stud- 

 ies show that the organization of embryonic 

 gonads with respect to sex type and capacity 

 for self-differentiation is acquired gradu- 

 ally, leading step by step to changes which 

 are stable and irreversible. Such transitions 

 coincide in some cases with distinct morpho- 

 logic events. In Willier's study of chick gon- 

 ads fixation of sex-type, with capacity for 

 autonomous differentiation, coincides with 

 the appearance of a distinct germinal epi- 

 thelium on the genital ridge. In rat gonads 

 (Torrey) the sexes differ greatly in this re- 

 spect; differentiation of prospective testes 

 becomes an autonomous process from the 

 first laying down of the medullary blastema, 

 whereas the ovary has but little capacity 

 for self-differentiation until much later, 

 after the appearance of a distinct cortical 

 zone. 



The fact that at certain stages of develop- 

 ment changes of an irreversible nature can 

 be demonstrated has led to the recognition 

 of so-called "critical periods," during which 

 rather abrupt transitions occur from a state 

 of lability to one of complete autonomy. 

 Thereafter, hormones or other extraneous 

 factors no longer have decisive effects. Such 

 stages have been demonstrated for various 

 types of sex primordia and are often nar- 

 rowly limited in time. In chick embryos 

 continued development of the Miillerian 

 ducts, or their involution, depends normally 

 on the ty{)e of gonad present, but at a cer- 



