92 CYTOPLASMIC STRUCTURES [CH 



forms. These spiral forms are an instance of the asymmetry 

 which is frequent, if not general, in the structure of sperma- 

 tozoa, and it has been suggested that the excentric position 

 of the nucleus of the unfertilised egg is a manifestation of 

 the same thing in the female gamete. The head consists 

 chiefly of a highly concentrated nucleus, which stains in- 

 tensely and almost evenly with chromatin stains. The 

 nucleus is probably enclosed in a thin sheath of cytoplasm, 

 although this is usually not readily visible. At its anterior 

 end there is often a pointed apical body or acrosome, the 

 function of which is probably to help in perforating the 

 surface of the egg when the spermatozoon enters it. The 



FIG. 14. Diagram of a spermatozoon, showing head with nucleus and aero- 

 some, middle- piece with end-knob, and tail with axial filament. (To save 

 space the tail is represented as bent on itself.) 



middle-piece is usually short and cylindrical, and contains 

 the base of the axial filament of the tail, which may be 

 swollen at the front end of the middle-piece, at its junction 

 with the head, into an " end-knob." It is this thickened base 

 of the axial filament which becomes, or at least produces, 

 the centrosome of the first segmentation division of the 

 fertilised egg. The tail is a long tapering filament, actively 

 motile like a flagellum, by the lashing of which the sperma- 

 tozoon swims vigorously in the seminal fluid or in water. 

 It consists of a thin outer sheath which stains only slightly, 

 surrounding a fine axial filament arising from the base in 

 the middle-piece and running throughout the length of the 

 tail. 



