HORMONES IX DIFFERENTIATION OF SEX 



89 



notwithstanding they have the genotype of 

 the opposite sex (for a discussion see Hum- 

 phrey, 1942). 



To insure invariable predominance of the 

 ovary in sex reversal it is usually necessary 

 to provide a marked advantage in size and 

 rate of development in favor of the female. 

 This can be done experimentally by resort- 

 ing to heteroplastic combinations (Fig. 2.9). 

 In parabiotic pairs composed of two species 

 of very different size {e.g., Amhystoma ti- 

 grinum-Amhy stoma maculatum) and with 

 a corresponding difference in growth rate, 

 when the members are of different sex the 

 larger species is almost invariably domi- 

 nant (Fig. 2.11; Burns, 1935). When the 

 large partner is a female the ovaries are 

 enormously larger than the testes of the 

 male and are always normal. The testes in 

 some cases undergo reversal almost from the 

 beginning of differentiation, and toward 

 metamorphosis are represented by very 

 small ovaries which contain a few well de- 

 veloped ovocytes. However, in most indi- 



viduals transformation sets in after consid- 

 erable differentiation has occurred, the testis 

 cords becoming hollowed out to form ovarial 

 sacs (Fig. 2.11) while the cortex persists and 

 grows rapidly. At metamorphosis males are 

 either completely transformed or the process 

 is far advanced. Complete transformation 

 of this type has also been reported by Wits- 

 chi (1937) in A. tigrinum-A. jefjersonianum 

 pairs. 



A similar result is obtained when single 

 gonad primordia are transplanted hetero- 

 plastically by Humphrey's method (Fig. 

 2.4) . In individuals bearing gonads of differ- 

 ent sex, when the ovary is of the larger spe- 

 cies it is dominant regardless of whether it 

 belongs to the host organism or was derived 

 from the graft. Histologically the reversal 

 process is the same as in parabiotic pairs 

 (Humphrey, 1935a, b). For fuller discus- 

 sions of species and racial differences as 

 they affect physiologic sex dominance and 

 the capacity of the gonads to undergo re- 

 versal in different species see Witschi (1934, 



B 



Fig. 2.9. Heteroplastic combinations uniting diiierent species of salamander {Amhystoma 

 tigrinum and A. punctatum) which differ greatly in eventual size and rate of growth. A. Ven- 

 tral view of paired embryos just after operation, showing fusion in the cervical region (punc- 

 tatum member at left). B. A pair after metamorphosis showing the great difference in size; 

 the larger animal is the tigrimnii member. 



