428 



HISTOLOGY 



nebenkern or accessory nucleus, a body found in the spermatid. The 

 middle-piece contains a round body which is sometimes double and is 

 called the end-knob. It is chromatic, and lies just behind the nucleus. 



Apical body or acrosome. 



Nucleus. 



End-knob. 



Middle-piece. 



Envelope of the tail. 

 Axial filament. 



End-piece. 



FIG. 381. Labeled diagram of a typical flagellate spermatozoon, and figures of eleven actual 

 spermatozoa to typify some of the principal forms. A, the badger, Meles; B, a bat, Vesper- 

 ugo; C, opossum, Didelphys ; D, a bird, Muscicaps ; E, a sturgeon, Acipenser ; F, a crab, 

 Porcdlana; G, the lobster, Homarus ; H, a crustacean, Polyphemus / /, an insect, Calathus ; 

 J, a salamander, Triton; K, a snake, Coluber. (From WILSON after WILSON (K and C); 

 BALLOWITZ (A, B, D, E, 7, and J); ZACHARIAS (H); GROBBEN (F); and HERRICK (G).) 



There is often a ring-shaped structure associated with it. The middle- 

 piece is sometimes much larger than the head (Fig. 381, B). 



Running distally from the end-knob, in our typical spermatozoon, is 

 a thread-like structure known as the axial filament. This filament 

 becomes continuous, at the end of the middle-piece, with the central 

 portion of a single, long flagellum. 



