THE SEMEN AND SPEEMATOZOA 



371 



contains the fluid secretions of the accessory genital glands and 

 many coarse protoplasmic seminal granules. When first formed 

 the spermatozoa are motionless, but when the secretion of the 

 testes is diluted with the secretions of the seminal vesicles and 

 prostate they become very active and re- 

 tain their motility for a long time even 

 under the most unfavorable conditions. 



The spermatozoa are flagellate bodies 

 which consist of a head, a middle piece, 

 and a tail. Each of these subdivisions is 

 a composite structure. The head of the 

 human spermatozoon is an ovoid body 

 about 4.5 to 5 ^ long, 3 to 3.5 /x, broad, and 

 0.5 to 1 ^ thick. When seen " on the flat " 

 (surface view) its outline is oval but when 

 viewed in profile it has a pyriform shape. 

 It is united to the middle piece by a slight- 

 ly constricted neck and its anterior ex- 

 tremity forms a blunt point analogous to 

 the " lance " of the spermatosomes of cer- 

 tain of the lower vertebrates. The head 

 of the human spermatozoon consists of 

 two distinct portions, a posterior solid 

 mass of chromatin which stains deeply 

 with nuclear dyes, and an anterior clear 

 portion or head cap. This head cap in 

 some of the mammalian species contains 

 a rounded chromatic body, the acrosome. 



At the posterior pole of the nuclear 

 chromatin, where it joins the middle piece, 

 is a small dark dot, the inner centrosome, 

 from which the axial filament, beginning 



with a minute end knob, is continued backward through the mid- 

 dle and tail pieces and is inclosed by a dedicate cytoplasmic en- 

 velope. At the tip of the tail piece the axial fibre is no longer 

 surrounded by cytoplasm but projects as a naked end piece. The 

 flagellum varies in diameter from a breadth of about 1 /*. in the 

 middle piece, to the immeasurably delicate end piece ; in length it 

 is about 45 to 50 //,. 



In the spermatozoa of the lower animals still other structures 

 may be demonstated in the flagellum, chief among which are (1) 



FIG. 301. SPERMATOSOMES 

 FROM THE SEMEN OF MAN. 



A, usual type ; 5, " giant " 

 double spermatozoon. Hem- 

 atein and eosiii. x 1200. 



