138 



BIOLOGIC BASIS OF SEX 



of the duct is always present, it is suscepti- 

 ble to inhibition by male hormone while the 

 tubo-uterine portion is never so affected 

 (Burns, 1942b). Thus, specific morphologic 

 defects which appear in experimental re- 

 sults can often be directly related to devel- 

 opmental peculiarities of the species in 

 question and in final analysis are an expres- 

 sion of constitutional factors. 



However, constitutional differences which 

 control the morphologic representation of 

 sex primordia do not as a rule involve sim- 

 ply the presence or the absence of a part, 

 but more commonly have a quantitative ex- 

 pression, affecting the extent to which the 

 structures are developed or the length of the 

 period during which they are present and 

 capable of responding. An example is found 

 in the gonads of birds, in which there are 

 marked lateral differences between right 

 and left sides (p. 95). In consequence, the 

 effects of hormones on the right and the left 

 gonads may be different, not qualitatively 

 but in degree. The left ovary, with its 

 strongly developed cortical component (Fig. 

 2.12), is only moderately affected by doses 

 of male hormone which almost completely 

 transform the rudimentary right gonad 

 (Willier, 1939). The morphologic differences 

 between right and left testes are less 

 marked, but consistently the germinal epi- 

 thelium is better developed and tends to 

 survive longer on the left side than on the 

 right. Consequently, female hormone read- 

 ily transforms a left testis into an ovotestis, 

 and with stronger dosages into an almost 

 normal ovary, but the right testis is but 

 slightly affected except when the dosage is 

 very large. It appears also from studies of 

 the effects of graduated dosages that thres- 

 hold differences for the two sides may be 

 involved, thus a physiologic as well as a 

 morphologic factor is introduced. It is not 

 held that experimental failures or anomalies 

 are always directly traceable to specific 

 morphologic deficiencies; however, the fre- 

 quency with which such correlations appear 

 indicates the importance of underlying 

 structural variations in modifying the re- 

 sponses of sex primordia under experimental 

 conditions. 



B. CONSTITUTIONAL FACTORS AND PHYSIOLOGIC 



DIFFERENCES IN THE ORGANIZATION 



OF SEX PRIMORDIA 



In the foregoing cases obvious morpho- 

 logic differences provide, at least in part, a 

 basis for observed differences in the experi- 

 mental behavior of sex primordia. It is un- 

 likely that morphologic differences of this 

 order exist without an underlying physio- 

 logic differentiation. On the other hand, un- 

 der experimental conditions, physiologic dif- 

 ferences often become apparent which have 

 no visible morphologic expression, as in the 

 inhibition of the vaginal canals of female 

 opossums cited above. Certain accessory sex 

 structures in birds exhibit lateral differences 

 in sensitivity to hormones which are evi- 

 dently a reflection of the general tendency 

 to asymmetrical development in this group. 

 In normal females only the left Miillerian 

 duct develops into a functional oviduct ; the 

 right, although originally well developed, 

 regresses at an early stage. After castration, 

 however, both ducts develop equally (Fig. 

 2.25) as is also the case when the ducts are 

 isolated in vitro. Involution of the right 

 oviduct, then, is conditioned in some way 

 by the ovaries and it has been shown that 

 either the right or left ovary alone is ef- 

 fective (Wolff and Wolff, 1951). Evidently 

 the ovaries exert an inhibitory action on 

 the right oviduct which is not effective on 

 the left. Presumably a threshold difference 

 is involved.^*' 



Other examples may be cited. The syrinx 

 and the genital tubercle remain small, sym- 

 metrical, and essentially undifferentiated in 

 females, but in males they become large and 

 highly asymmetrical (Fig. 2.25). Unlike the 

 paired structures previously dealt with, 

 these organs are single and median in posi- 

 tion and the asymmetry of the male form 

 is due to unequal development of the lateral 

 halves of the organs. It is well established in 

 the case of eacii that the female hormone 



""It seems unlikely tliat the inhibitoiy factor in 

 this case is the female hormone since the introduc- 

 tion of estrogenic hormones into incubating eggs 

 causes persistence of the right oviduct. However, 

 the concentration or dosage may be a factor in this 

 curious effect. 



