22 



THE TESTIS AND ITS RELATION TO REPRODUCTION 



terone and dehydroisoandrosterone. These two androgens are not as powerful 

 as that prepared from testicular tissue. Testicular androgen was first isolated 

 from testicular tissue in 1935 and was given the name testosterone. It also 

 has been synthesized from cholesterol. It is the most powerful of the androgens 

 and probably similar, if not identical, with the substance produced in the 

 testis (Koch, '42). 



2. Activities of the Testis 

 a. Seasonal and Non-seasonal Types of Testicular Activity 



The testis has two main functions: the production of sperm and formation 

 of the male sex hormone. In many vertebrates these two activities represent 

 a continuous procedure during the reproductive life of the male animal. This 



Fig. II. Seasonal spermatogenesis and accessory gland development in the ground 

 squirrel, Citellus tridecemlineatus. Stippling below base line shows period of hibernation, 

 whereas crosshatching reveals the reproductive period. (From Turner: General Endo- 

 crinology, Philadelphia, Saunders, after L. J. Wells.) 



condition is found in certain tropical fish, in the common fowl and various 

 wild tropical birds, and in many mammals, such as man, the dog, bull, stallion, 

 cat, etc. On the other hand, in the majority of vertebrates these activities of 

 the testis are a seasonal affair. This condition is found in most fish, practically 

 all amphibia, all temperate-zone-inhabiting reptiles, most birds, and many 

 mammals. Among the latter, for example, are the ferret, deer, elk, fox, wolf, 

 and many rodents, such as the midwestern ground squirrel. Seasonal periodicity 

 is true also of the common goose and turkey. 



Sperm-producing periodicity is not correlated with any particular season, 

 nor is spermatogenesis always synchronized with the mating urge, which in 

 turn is dependent upon the male sex hormone. In some forms, these two 

 testicular functions may actually occur at different seasons of the year, as 

 for example, in the three-spined stickleback, Gasterosteus aculeatus (fig. 15). 

 (See Craig-Bennett, '31.) In general, it may be stated that sperm are produced 



