CH. LVII.] 



SPERMATOZOA. 



797 



The Spermatozoa, suspended in a richly albuminous fluid, 

 constitute the semen. Each spermatozoon is composed of three 

 parts, a head, a middle part, and a tail. The head varies in shape 

 in different animals, but in man it is oval, and pointed anteriorly. 

 The middle piece is short and cylindrical, with a spiral fibre pass- 

 ing round it. The tail is long, tapering, and vibratile ; its action 

 resembles that of a cilium, and gives to the spermatozoon its power 

 of locomotion. The end piece of the tail is described by Retzius 



Fig. 608. Spermatic fila- 

 ments from the human 

 vas deferens. i, magni- 

 fied 300 diameters; 2, 

 magnified 800 diameters ; 

 a, from the ride ; b, from 

 above. (From Kiilliker.) 



Fig. 609. Spermatozoa, i, of sala- 

 mander ; 2, human. (H. Gibbes.) 



as distinct from the rest, and in some animals is divided into two 

 or three fibrils. 



In some animals (newts, salamanders, &c.) a fine filament or 

 membranous expansion is attached to the tail in a spiral manner 

 (fig. 609). A similar appearance has been described by some 

 observers in mammalian spermatozoa. 



The spermatozoa are formed from the small spermatoblasts of 

 the third or innermost layer of the seminiferous tubules ; these 

 originate from the spermatogenic cells of the second layer, and 

 these from the lining cubical epithelium. When a lining cell 

 divides into two, one becomes a spermatogenic cell, the other 

 becomes elongated to form a sustentacular cell. 



In the conversion of a spcrmatoblast into a spermatozoon, the 

 nucleus forms the head ; the tail develops as a fine filament 



