STRUCTURE OF THE SPERMATOZOA. 177 



loose texture, and contains numerous lymphatic clefts, which form an 

 intercommunicating system of commencing lymphatic vessels. Lying 

 in this inter tubular tissue are strands of polyhedral epithelium-like 

 cells (interstitial cells) of a yellowish colour ; they are much more 

 abundant in some species of animals (cat, boar) than in others. They 

 accompany the blood-vessels before these break up to form the capillary 

 networks which cover the walls of the seminiferous tubules. 



FIG. 214. HUMAN SPERMATOZOA. 



1, in profile ; 2, viewed on the flat : b, head ; c, middle-piece ; d, tail ; e, end-piece of the tail. 



The spermatozoa. Each spermatozoon consists of three parts, a 

 head, a middle part or body, and a long tapering and vibratile tail. 

 In man (fig. 214) the head is of a flattened oval shape, somewhat more 

 flattened and pointed anteriorly ; and the middle-piece is short and 

 cylindrical, and appears to have a spiral fibre passing round it ; but in 

 different animals the shape of the head and the extent of the middle- 

 piece vary greatly. In the rat the head is long, and is recurved ante- 

 riorly ; it is set obliquely on the middle-piece, which is also of con- 

 siderable extent, and has a closely wound spiral filament encircling it 

 in' its whole length. The tail is the longest part of the spermatozoon, 

 and during life is in continual vibratile motion, the action resembling 

 that of the cilia of a ciliated epithelium-cell. 



The spermatozoa are developed from the small cells or spermato- 

 blasts which form the innermost strata of the seminal epithelium, and 

 are themselves produced by the division of the large cells of the second 

 layer. The nucleus of the spermatoblast forms the head of the sperma- 

 tozoon, while the protoplasm forms first the middle-piece and subse- 

 quently the tail ; but a portion of the protoplasm of each spermato- 

 blast containing a number of small darkly staining particles (seminal 

 granules) appears always to become detached and disintegrated before 

 the spermatozoon is fully mature. 



