THE TESTIS 375 



interstitial tissue is characterized by the presence of many pecul- 

 iar ovoid interstitial cells of large size, whose significance is not yet 

 determined. They contain a large nucleus which has a distinct 

 nuclear membrane, a reticular chromatin network, and many 

 karyosmes. Their cytoplasm is reticular and finely vacuolated. 



The seminiferous tubules begin at the periphery of the organ 

 with either a blind extremity or very frequently a peripheral loop 

 formed by the anastomosis of adjacent tubes. They pursue their 

 way through the lobule in an extremely tortuous manner (tortuous 

 or serpentine tubule), and finally near the apex of the lobule 

 become relatively straight (tubuli recti). They then enter the 

 mediastinum and by frequent anastomoses form the rete testis. 

 The tortuous portion of the seminiferous tubule is relatively long, 

 its straight portion very short. It is the former which is of the 

 greater physiological importance and which is to be considered as 

 the true seminiferous tubule, for it is here that the spermatozoa, 

 the essential elements of the semen, are produced. The straight 

 tubules mark the beginning of a system of excretory ducts which 

 include the ductuli efferentes, epididymis, vas deferens, ejactula- 

 tory duct, and urethra. 



The tortuous seminiferous tubules are lined by a peculiar form 

 of epithelium, which, since it consists of several layers of sphe- 

 roidal cells, might be described as a stratified spheroidal type. 

 The tubule is invested by a very delicate tunica propria upon 

 whose homogeneous basement membrane the epithelium rests. 



The cells of the lining epithelium are divisible into three 

 types, which are from without inward : 1, a single layer of small 

 cuboidal spermatogonia ; 2, one or two rows of very large sperma- 

 tocytes ; 3, and three to five rows of spheroidal spermatids. Besides 

 these a fourth type of cell occurs at fairly regular intervals in the 

 circumference of the tubule ; these are the so-called Sertoli's cells. 

 These last cells are of columnar form, rest upon the membrana 

 propria and extend inward for a variable distance, sometimes 

 penetrating as far as the innermost cell layers. The developing 

 spermatozoa are, at a certain stage, intimately united with the 

 central ends of the Sertoli cells, the resulting group a supporting 

 cell of Sertoli with its attached spermatozoa forming the so- 

 called spermatoblasts * of von Ebner \. 



* Much confusion has arisen through the use of this term by certain authors 

 as synonymous with the term spermatid. 



f Untersuch. a. d. Inst. f. Physiol. u. Hist., Graz., 1871. 



