ANATOMICAL FEATURES 17 



sperm form. The Sertolian cells thus may act as nursing elements during 

 sperm metamorphosis. 



The spermatogenic cells or spermatogonia (germinal epithelium of the 

 tubule) lie toward the outer portion of the seminiferous tubule between the 

 various Sertoli elements. As a rule spermatogonia lie apposed against the base- 

 ment membrane of the tubule (see fig. 8 and Chap. 3). 



b. Interstitial Tissue 



The interstitial tissue of the testis is situated between the seminiferous 

 tubules (fig. 8). It consists of a layer of connective tissue applied to the 

 basement membrane of the seminiferous tubule and of many other structures, 

 such as small blood and lymph vessels, connective tissue fibers, connective 

 tissue cells, mast cells, fixed macrophages, etc. The conspicuous elements of 

 this tissue are the so-called interstitial cells or cells of Leydig (fig. 8). In 

 man, cat, dog, etc., the cells of Leydig are relatively large, polyhedral ele- 

 ments, possessing a granular cytoplasm and a large nucleus. 



5. The Testis of Vertebrates in General 



In the vertebrate group, the testis shows marked variations in shape and 

 size. In many fishes, the testes are irregular, lobular structures, but in other 

 fishes, amphibia, reptiles, birds, and mammals, they assume an ovoid shape. 

 The size of the testis is extremely variable, even in the same species. The 

 testis of the human adult approximates 4 to 5 cm. in length by 3 cm. wide 

 and weighs about 14 to 19 Gm. The testis of the horse averages 11 cm. 

 long by 7 cm. wide with a weight of 30 to 35 Gm., while that of the cat is 

 1.6 cm. long and 1.1 cm. wide with a weight of 1.5 Gm. In the mud puppy, 

 Necturus, the testis is approximately 3.5 cm. long and 0.8 cm. wide with a 

 weight of 0.3 Gm. The testis of the large bullfrog is 1.2 cm. by 0.5 cm. with 

 a weight of 0.8 Gm. In comparison to the foregoing, Schulte ('37) gives the 

 weight of each testis of an Indian elephant as two kilograms! 



Regardless of size or shape, the presence of seminiferous tubules and inter- 

 stitial tissue may be observed in all vertebrate testes. In some species the 

 seminiferous tubule is long; in others it is a short, blunt affair. The interstitial 

 cells may be similar to those described above, or they may be small, incon- 

 spicuous oval elements. 



6. Accessory Reproductive Structures of the Male 



a. The Reproductive Duct in Forms Utilizing External Fertilization 



The accessory reproductive organs of the vertebrate male are extremely 

 variable in the group as a whole. A relatively simple reproductive duct (or 

 in some no duct at all) is the rule for those forms where fertilization is 

 effected in the external medium. In cyclostome fishes, for example, the repro- 

 ductive cells are shed into the peritoneal cavity and pass posteriad to emerge 



