Chap. XXXi.] THE MALE GENITAL ORGANS. 249 



groups the head, i.e., the thin end containing the 

 flattened homogeneous nucleus, is directed towards 

 the inner seminal cells, while the opposite extremity 

 is directed into the lumen of the tube. Meanwhile 

 the inner seminal cells continue to divide, and thus 

 the groups of young spermatozoa get more and more 

 buried, as it were, between them. 



326. The original cell-body of the spermatoblasts 

 goes on elongating until its protoplasm is almost, but 

 not quite, used to form "the rod-shaped middle piece 

 (Schweigger Seidel) of the spermatozoa; from the 

 distal end of this, a thin long hair-like filament, called 

 the tail, grows out. Where this joins the end of the 

 middle piece, there is, even for some time afterwards, 

 a last remnant of the granular cell-body of the original 

 spermatoblast to be noticed. 



When the granular interstitial substance holding 

 together the spermatozoa of a group has become dis- 

 integrated, the spermatozoa are isolated. While this 

 development of the spermatozoa goes on, the inner 

 seminal cells continue to produce spermatoblasts, 

 which in their turn are converted into spermatozoa. 



327. Spermatozoa (Fig. 139). Fully formed 

 spermatozoa of man and mammals consist of a homo- 

 geneous flattened and slightly convex-concave head (the 

 nucleus of the original spermatoblast), a rod-shaped 

 middle piece (derived directly from the cell body of 

 the spermatoblast), and a long hair-like tail. While 

 living, the spermatozoa show very rapid oscillatory and 

 propelling movement, the tail acting as a flagellum or 

 cilium ; its movements are spiral. 



In the newt there is a fine spiral thread 

 attached to the end of the long, curved, spike-like 

 head, and by a hyaline membrane it is fixed to the 

 middle piece ; it extends beyond this as the long thin 

 tail. Also in the mammalian and human sperma- 

 tozoa, a similar spiral thread, closely attached to the 



