90 



BIOLOGIC BASIS OF SEX 



1957). Humphrey (1942), and Gallien 

 (1955). 



The progress of sex reversal, and the 

 mechanism by which the transformation is 

 effected, can be analyzed histologically only 

 when it takes place as a secondary process, 

 after a certain amount of sex differentiation 

 has previously occurred. In this case both 

 histologic components are distinctly repre- 

 sented but the normally recessive compo- 

 nent seeks to become dominant; ovaries 

 become testes by regression of the differenti- 

 ated cortex accompanied by growth and dif- 

 ferentiation of the medullary element (Figs. 

 2.8 and 2.10), and testes are converted into 

 ovaries by the reverse process (Figs. 2.6C 

 and 2.11A-D). The mechanism is flexible, 

 however, and there is much variability, even 

 among individuals in the same experiment, 

 with respect to the stage at which reversal 

 sets in, its progress, and its final outcome. 

 In some cases removal of the dominant go- 

 nad after reversal is far advanced may be 

 followed by a second reversal toward the 

 original sex (Humphrey, 1942). 



However, transformation is not always a 



secondary process, set in action only after 

 a certain amount of differentiation has al- 

 ready occurred; as noted above, when the 

 ciuantitative disparity between the interact- 

 ing gonads is sufficiently marked reversal 

 may proceed from the earliest stages of dif- 

 ferentiation. In this case the term "reversal" 

 is less apt since there were no previous his- 

 tologic steps to be retraced. Transformation 

 is indicated chiefly by the unbalanced sex 

 ratios at the end of the experiment; but in 

 certain cases it is confirmed by character- 

 istic histologic peculiarities (Burns, 1935, 

 Fig. 28; Witschi, 1937, Fig. 39). In the het- 

 eroplastic grafting experiments of Hum- 

 phrey, on the other hand, direct proof is 

 available since in many cases the sex of the 

 grafted gonad, although conforming with 

 that of the host, differs from the sex of the 

 donor animal which is reared to provide a 

 direct control. 



Although primary reversal of sex differ- 

 entiation, as described above, occurs more 

 readily when a marked disparity in size 

 leads to early dominance, it may also occur 

 under conditions which greatly retard de- 



H^iH^ 



Fig. 2.10. Two views of an ovary of Atnbystoma tigrinuin uudcigoing reversal under the 

 influence of the testes of a male partner. The cortical zone, witli characteristic early ovo- 

 cytes, is still prominent; however, medullary development is i)roceeding and in tlie region 

 rei^resonted in B, testis lobules are forming. (From l^ K. Buiiis, J. Exper. Zool., 55, 123-129, 

 1930; 60,339-387, 1931.) 



