84 



BIOLOGIC BASIS OF SEX 



NDIFFERENT GONAD 



OVARY TtSTIS 



Fig. 2.5. Diagrammatic representation of the 

 male and female components of the sexually un- 

 differentiated amphibian gonad and their roles in 

 sex differentiation: the medulla is stippled, the 

 cortex is plain. The broken arrows indicate the 

 mutually antagonistic or inhibitory actions exerted 

 between the two components in the course of sex- 

 ual differentiation (Witschi). 



mass, the medulla, which has the develop- 

 mental potentiality of a testis. Surrounding 

 the medulla is a peripheral zone, the cortex, 

 which is specifically female in potency. The 

 topographic relationships of medulla and 

 cortex are illustrated diagrammatically in 

 Figure 2.5, and as they appear histologically 

 in male salamanders in Figure 2.QA and B. 

 Male and female potentialities are evidently 

 pre-established in the medullary and corti- 



cal components at an early stage since final 

 differentiation as a testis or an ovary does 

 not involve the transformation of one sex 

 comj^onent into the other but rather the 

 gradual predominance of one element and 

 the recession of the other. 



Not only are the two components dis- 

 tinctly segregated in the indifferent gonad, 

 they have separate origins. It has long been 

 recognized that the medulla in both sexes 

 is derived from the mesonephric blastema 

 in the form of a series of cellular strands, 

 the medullary cords or rete cords, which 

 grow into the genital ridge at an early stage. 

 At first similar in appearance in the two 

 sexes, their later differentiation follows very 

 different patterns. In the ovary the cords 

 expand distally, forming a series of saccular 

 cavities, the ovarian sacs. Most of the germ 

 cells are excluded from these sacs and come 

 to lie in a peripheral layer beneath the peri- 

 toneal epithelium covering the gonad. This 

 zone becomes the cortex. In prospective 

 testes the cords branch and proliferate rap- 

 idly, enveloping and incorporating the ma- 

 jority of the germ cells in a compact central 

 mass, the medulla ( Fig. 2.6 ; for a fuller de- 

 scription see Willier, 1939). Thus the rela- 

 tive proportions of cortex and medulla in 

 the sexes depend on the pattern of differen- 



FiG. 2.6. Sections showing the histologic appearance of cortex and medulla in early larval 

 stages under various conditions. A. Normal testis of an Amby stoma tigrinum larva; note the 

 thin cortical zone, comparable to a germinal epithelium, which covers most of the surface. 

 B. The cortex of an intersexual testis dissected free from the medullary core, from the punc- 

 tatum member of an Ambtjstoma tigrinum, 9 ; A. Punctatum, $ pair (c/. Burns, 1935, plate 

 4). C. Intersexual testis of the male member of a tigrinum-tigrinum pair, showing tl)e rela- 

 tive development of the medullary and cortical components (see Burns, 1930). 



