100 GENERAL EMBRYOLOGY 



Through the middle of the tail is an axial filament connect- 

 ing with the centrosome of the middle piece, a relation 

 which is very common in flagellate or ciliate structures. 

 Proximally the axial filament passes through a ring-like 

 structure, the annulus, in the end of the middle piece. 

 Distally the filament may continue beyond the cytoplasmic 

 envelope of the tail as a terminal filament or end piece. There 

 is the greatest variety in details of form of the sperm, affecting 

 chiefly the form of the head and acrosome, length of tail, and 

 total size; a few of the more common or more striking forms 

 are illustrated in Fig. 52. (In cases where a neck region is 

 distinguished, the middle piece is often regarded as the proximal 

 part of the tail.) 



The smallest spermatozoa are found in Amphioxus and 

 are only 0.016-0.020 mm. (16-20 micro) in length; the largest 

 are found in some of the Amphibia where in Salamandra they 

 are about 0.7 mm. (700 micra) in length, and in Discoglossus 

 2.0 mm. (2000 micra) , the maximum length known. The sperm 

 cells of the Crustacean Cypris, are also of this gigantic size 

 (2 mm.). The spermatozoa of most of the Vertebrates are 

 25-75 micra in length. The human spermatozoon (Fig. 51) 

 represents about the average size; the dimensions of this are 

 (Krause): total length, 52-62 micra (1/400-1/500 inch); length 

 of head, about 4.5 micra (between 1/5000 and 1/6000 inch) ; 

 length of middle piece, about 6 micra (about 1/4200 inch); 

 length of tail piece, 41-52 micra (1/500 to 1/600 inch); width 

 of head 2-3 micra (1/12000 to 1/8000 inch); thickness of head, 

 (this is one of the few instances where the head is flattened), 

 about 1 micron (1/25000 inch). 



The number of sperm formed by a single individual is very 

 large in most organisms and can be only roughly estimated. 

 It has been computed (Lode) that the total number formed in 

 man may average about three hundred and forty billion, or 



Ethusa (Grobben). N. The Crustacean, Inachus (Grobben). O. The Crusta- 

 cean Stda (Wjnsniann) . P. The Crustacean, Bythotrephes (Weismann). k, end 

 knc ; m, middle piece; n, nucleus; p, perforatorium; u, undulatory membrane. 

 Not drawn to same scale. A-F, I-K, from Wilson. 



