6 THE TESTIS AND ITS RELATION TO REPRODUCTION 



1. Anatomical Location of the Testis 

 In most vertebrates other than mammals, the testes are suspended well 

 forward within the peritoneal cavity. In the Mammalia, however, the con- 

 dition is variable. In the monotrematous mammals, Echidna and Ornithoryn- 

 chus, the testes are located within the peritoneal cavity near the kidneys. In 

 the elephant the testes also are located in this area. Schulte ('37) describes 

 the position of the testes in an Indian elephant (Elephas indicus), 20 years 

 old, as being "retroperitoneal lying on each side medial to the lower pole of 

 the kidney." (The kidneys were found to lie retroperitoneally on either side of 

 the lower thoracic and lumbar vertebrae, and each measured about 275 mm. 

 in length.) However, in the majority of mammals the testes descend pos- 

 teriad from the original embryonic site, the extent varying with the species. 

 In some there is a slight posterior migration, and the testes of the adult are 

 situated well forward in the pelvic region. Examples of this condition are 

 found in conies, whales, sea cows, African jumping shrews, and in arma- 

 dillos. In sloths and American anteaters, the testes may descend into the 

 pelvic cavity and lie in the area between the urinary bladder and the posterior 

 body wall. However, in most of the eutherian and marsupial mammals, a 

 dual outpushing of the postero-ventral body wall occurs into which the testes 

 come to lie either permanently, or, in some forms, temporarily during the 

 breeding season. This outward extension of the body-wall tissues is known 

 as the scrotum; it involves not only the skin, muscle and connective tissues 

 of the body wall but the peritoneal lining as well (fig. 2). (The interested 

 student may consult Weber ('28) and Wislocki ('33) for data concerning 

 the extent of testis descent in mammals.) 



The peritoneal evaginations into the scrotal sac are two in number, one 

 for each testis; each evagination is known as a processus vaginalis (figs. 3E, 

 F; 4A, B). In many mammals this evagination becomes separated entirely 

 from the peritoneal cavity, and the testis, together with a portion of the 

 sperm-conveying duct, lies suspended permanently in a small antechamber 

 known as the inguinal bursa or serous cavity of the scrotum (fig. 4B). (See 

 Mitchell, '39.) This condition is found in the horse, man, opossum, bull, 

 ram, dog, cat, etc. In certain other mammals, such as the rat, guinea pig, and 

 ground hog, the inguinal bursa does not become separated from the main 

 peritoneal cavity, and a persistent inguinal canal remains to connect the in- 

 guinal bursa with the peritoneal cavity (fig. AC). In some rodents the testes 

 pass through this persisting inguinal canal into the scrotum as the breeding 

 season approaches, to be withdrawn again after the breeding period is termi- 

 nated. The ground squirrel, Citellus tridecemlineatus (Wells, '35) and the 

 ground hog, Marmota monax (Rasmussen, '17) are examples of mammals 

 which experience a seasonal descent of the testis. 



In the majority of those mammals possessing a scrotum, it is a permanent 

 structure. In a few, however, it is a temporary affair associated with the 



