STRUCTURE OF THE VERTEBRATES 



267 



Descent of the testes. The primitive position of the testes is 

 within the bod}''; but in the marsupials and the majority of 

 placentals the testes descend into a scrotal sac which is homolo- 

 gous with the external labia of the female. The testes do not 

 descend in the elephants, whales and sirenia, but this is ev- 

 idently a specialization. In other mammals the sperm cells will 

 not mature when kept in the body cavity. Descent of the testes 

 is seasonal in many rodents, the glands remaining within the 

 body except during the mating season ; in some the testes may be 

 withdrawn at will from the scrotum; while in others the connec- 

 tion between the scrotum and the body cavity is lost and the 

 testes remain in position after descent. 



The gonads arise in a retro-peritoneal position and are cov- 

 ered by the somatic layer of the peritoneum. At the posterior end 



Coelomic' 

 cavity 



Testis 



Peritoneum 



GubemcLCulum 



Coelomic 

 cavity 



Vas deferens- 

 Scrotw. 



Testis 



Tunic of 

 testis 



Fig. 149. Descent of the Testes in the Mammal. As the testes descend from 



their body position, a pocket of the coelomic cavity is drawn into the 



scrotum. In some mammals the connection with the body cavity remains 



open as in (B) ; the connection is lost in man. (C). 



of the embryonic gland there is a gubernacular ligament which 

 passes from the testis to the scrotum. As growth proceeds the 

 testes are drawn posteriorly, and the peritoneal covering is 

 carried with them as the tunic of the testes. Shortly before (or 

 after) birth in the human the testes pass through the openings 

 of the body wall into the scrotum. The inguinal canals through 

 which the glands pass are located in the groin, and the testes 

 leave the body anterior and ventral to the pubic bones, passing 

 between the pelvis and the skin. A pocket of the coelomic cavity 

 is carried into the scrotum with them, and the somatic mesoderm 



