80 



BIOLOGIC BASIS OF SEX 



A B C 



Fig. 2.2. Diagram illustrating different modes of grafting in amphibians in order to bring 

 about vascular continuity between individuals, and association of gonads of different sex. A. 

 Homoplastic twins in salamanders. The body cavities are largely separated and vascular com- 

 munications between the gonads are remote. For combination of dissimilar species see Fig. 

 2.9B. B. Anuran twins, showing side-to-side or head-to-tail union; reversal changes appear 

 only under the first condition, when the gonads are in close proximity. (After E. Witschi, in 

 Sex mid Internal Secretions, The Williams & Wilkins Company, 1932). C. Orthotopic trans- 

 plantation of the gonad primordium by the method illustrated in Fig. 2.4, resulting in two 

 gonads of opposite sex resident in a single individual (Humphrej''s method). 



greatly, depending on the species under 

 study and on various experimental condi- 

 tions, as will appear later. 



The grafting of gonads alone (as opposed 

 to the union of whole organisms) can be car- 

 ried out in embryonic stages of development 

 or in early larval life. The latter method 

 was tried first. The gonads, attached to a 

 segment of the mesonephric bodies, were re- 

 moved from young larvae at or soon after 

 the onset of sex differentiation and inserted 

 into the body cavity of older larvae (Burns, 

 1928) . The development and activity of such 

 grafts depends on the extent to which they 

 become attached and vascularized. When 

 graft and host are of different sex the grafts 

 typically become intersexual, developing the 

 structure of ovotestes; and when a large and 

 well differentiated graft is in close proximity 

 to the gonads of the host the latter may be 

 similarly modified (Fig. 2.3). This method 

 of grafting has the disadvantage, however, 

 that reversal is usually incomplete, and 

 when graft and host gonads show reciprocal 

 modification it is sometimes difficult to de- 

 termine the primary sex of either. 



The method described above was soon 

 greatly improved upon by the development 

 of a technique for transplanting, at an ear- 

 lier stage, the prospective gonad-forming 

 tissue from one embryo to another (Hum- 

 phrey 1928a, b). At first such grafts were 

 placed in ectopic locations, but later it was 

 found advantageous to place them in the 

 normal (orthotopic) position in an embryo 

 from which the corresponding gonad pri- 

 mordium had been excised (Fig. 2.4). After 

 such an operation the host embryo bears on 

 one side its own gonad and on the other a 

 gonad which, in approximately half of all 

 cases, has come from an embryo of the 

 other sex (Fig. 2.2C). This method has im- 

 portant advantages over those previously 

 described. A single embryo bearing an or- 

 thotopic graft survives better and is more 

 easily reared tlian are parabiotic pairs, and 

 gonads grafted in tlio orthotopic position 

 usually develoj) better than in foreign sur- 

 roundings. Most important of all, the donor 

 embryo may be reared, thus establishing 

 with certainty the original sex of the grafted 

 gonad. This precise method has yielded un- 



