108 



BIOLOGIC BASIS OF SEX 



Fig. 2.19. A. The testis of a normal male opossum aged 20 days for comparison with that 

 of another male, B, treated for 20 days with a low dosage of estradiol dipropionate as de- 

 scribed in the text. Only a remnant of testis structure survives as a nodular mass in the 

 medullary region, representing straight tubules at the point of junction with the rete canals. 

 Note the well developed cortical zone witli numerous germ cells and a heavy germinal epi- 

 thelium. 



greater than in the preceding experiment. 

 It is not clear whether this is due mainly to 

 the lowered dosage or to what extent it is a 

 result of multiplication of ovogonia present 

 in smaller numbers in the younger gonads 

 (see Fig. 2.195). In any case, dosage in 

 some manner influences the survival and 

 multiplication of the gonia. Although the 

 cortex of the transformed testis is always 

 well developed, there is great variability in 

 the extent to which testicular structures 

 have survived in the medullary zone. Some 

 gonads exhibit well preserved male sex cords 

 and present the picture of a typical ovotes- 

 tis, whereas in others (Fig. 2.21B} only 

 traces of the male elements remain in the 

 form of degenerate sex cords and patches 

 of fibrous tissue. In these cases transforma- 

 tion is all but complete. 



Since this work is still in progress inter- 

 pretation must be tentative. It seems that 

 the primary effect of the female hormone 

 is a strong inhibition of the testis, affecting 

 both the primary sex cords and the inter- 

 stitium. Both influences are apparent at an 

 early stage (Fig. 2.17). Inhibition of testicu- 



lar differentiation i:)resumably i-)ermits sur- 

 vival of the germinal epithelium, to be 

 followed later by a renewal of activity pro- 

 ducing secondary sex cords and the cortex. 

 This course of events may simply be the 

 result of release from an inhibition nor- 

 mally imposed by the differentiating testis. 

 Counterparts are seen, for example, in the 

 spontaneous development of the medullary 

 component in transplanted rat ovaries when 

 cortical differentiation is interfered with, in 

 the development of the rudimentary cortex 

 in Pleurodeles after the medulla has been 

 suppressed by testosterone (p. 94), or in 

 the development of the heterotypic sex com- 

 ponent after castration in the toad or the 

 newly hatched chick. In the light of cur- 

 rent knowledge regarding the secretory 

 capacity of the embryonic testis (for dis- 

 cu.ssions see Jost, 1953, 1957; Burns, 

 1955b, and later in this chapter) it is proba- 

 ble that the primary condition for survival 

 of the germinal epithelium is suppression of 

 the interstitial tissue; cortical differentia- 

 tion is presumably the consequence of es- 

 cai)e from an inhibition normally exerted 



