REPRODUCTIVE ORGANS IN THE MALE- 285 



tile tail. In man the head is a flattened ovoid, ap- 

 pearing pear-shaped or pointed in one view and 

 rounded in another. It contains the nucleus and 

 stains heavily with nuclear dyes. It measures about 

 0.0045 mm. long, 0.0025 mm - broad, and 0.0015 mm. 

 thick. 



The middle piece, or body, is cylindrical in man, and 

 measures about 0.006 mm. in length, and o.ooi mm. 

 in thickness. In some animals, as the rat, a spiral 

 thread can be seen coiled about the periphery, 

 whilst through its center a slender filament seems to 

 pass and be continuous with the central filament of 

 the tail. This filament ends in a terminal enlarge- 

 ment placed in close proximity to the nucleus and 

 known as the terminal globule. 



The tail is about 0.045 mm. long and tapers toward 

 the extremity, ending in an extremely delicate fiber, 

 the end piece. Again, in the rat this end piece 

 seems to be the terminal part of the central filament 

 of the middle piece, which thus extends the whole 

 length of the tail. The spermatozoon is propelled 

 forward by a spirally lashing movement of the tail, 

 similar to the movement of cilia. Movement of 

 cilia, however, ceases as soon as they are removed 

 from their cell, which is not the case with the tail of a 

 spermatozoon, in which motion seems to be an in- 

 trinsic quality. So long as spermatozoa remain in 

 the male passages they are inert, but become active 

 as soon as expelled. 



Spermatozoa differ a great deal in the different 

 species of animals. They are very hardy cells, and 

 in the female passages may live for days and even 

 weeks. In some domestic birds, as turkeys, they 



