440 COMPARATIVE ANATOMY 



egg and is only exceptionally modifiable. Nevertheless, ovaries and 

 testes develop from morphologically similar genital folds, located between 

 the mesonephroi and the mesentery. At their first appearance, the genital 

 folds are elongated masses of epithelial cells, which become differentiated 

 into an external many-layered epithelium and an inner epithelial mass 

 derived from the peritoneum. Together with the mesonephros, the 

 gonad forms a urogenital ridge, the prominence of which is increased by 

 the growth of underlying adrenal tissue. (Fig. 367) 



As the genital folds increase in size, longitudinal grooves develop 

 separating them from the lateral mesonephros and the median mesentery. 

 Connexion of the gonad with the mesonephros and with the body- wall 

 is finally reduced to a thin mesentery-like membrane. In the male, this 

 membrane forms the mesorchium and in the female the mesovarium 

 through which blood and nervous connexions are retained. The gonads 

 remain undifferentiated in a ten to twelve millimeter embryo. 



Histogenesis of the Testis. By the time the male embryo has reached 

 a length of 13 mm., the gonad assumes characteristics which differentiate 

 it as a testis. One of these is the connective tissue tunica albuglnea 

 beneath the peritoneal epithelium. Within the medullary region of 

 the gonad, branched and anastomosing testis cords appear, separated by 

 less compact intermediate cords. Two kinds of cells may soon be seen 

 within the testis cords, primordial germ-cells and indifferent cells. 

 Opinion is divided whether the testis cords are formed by ingrowths from 

 the peritoneum or by concentration of epithelial cells within the medulla. 

 Uncertainty also prevails as to the source of the primordial germ-cells. 

 Some assert that they are derived from cells which migrate into the 

 gonad from the intestine. Others claim that they are peritoneal cells 

 which have entered the medulla. The latter opinion seems to have the 

 greater support. The fate of these cells is equally in doubt. Some 

 assume that they are the progenitors of the definitive germ-cells, the 

 spermatogonia, while others believe that they disappear and that the 

 indifferent cells become spermatogonia. 



The testis cords converge towards the mesorchium as a center. At 

 the base of the mesorchium, they combine to form the tubules of the 

 rete testis. These in turn unite with tubules of the mesonephros. Finally 

 the testis cords become hollow and differentiated as seminiferous tubules. 

 Epithelial cells within the intermediate cords form interstitial cells 

 which are believed to have an endocrinal function. (Fig. 368) 



Histogenesis of the Ovary. Differentiation of the ovary is slower 

 than that of the testis. The female embryo is from ten to eleven weeks 

 old (50 mm.) before ovarian characteristics appear. In contrast with 

 the testis, the ovary lacks a tunica albuginea and epithelial cords. At 

 three to five months, the primordial germ-cells in the medullary region 



