THE SPERMATOZOON 103 



ever, vary more or less widely from it (Figs. 48-50). The head 

 (nucleus) may be spherical, lance-shaped, rod-shaped, spirally twisted, 

 hook-shaped, hood-shaped, or drawn out into a long filament; and 

 it is often^.divided into an anterior and a posterior piece of different 

 staining capacity, as is the case with many birds and mammals. 

 The apex sometimes appears to be wanting e.g. in some fishes 

 (Fig. 48). When present, it is sometimes a minute rounded knob, 

 sometimes a sharp stylet, and in some cases terminates in a sharp 

 barbed spur by which the spermatozoon appears to penetrate the 

 ovum (Triton). In the mammals it seems to be represented by a 

 cap-like structure, the so-called " head-cap," which in some forms 

 covers the anterior end of the nucleus. It is sometimes divided into 

 two distinct parts, a longer posterior piece and a knob-like anterior 

 piece (insects, according to Ballowitz). 



The middle-piece or connecting-piece shows a like diversity 

 (Figs. 4850). In many cases it is sharply differentiated from 

 the flagellum, being sometimes nearly spherical, sometimes flattened 

 like a cap against the nucleus, and sometimes forming a short 

 cylinder of the same diameter as the nucleus, and hardly distin- 

 guishable from the latter until after staining (newt, earthworm). 

 In other cases it is very long (reptiles, some mammals), and is 

 scarcely distinguishable from the flagellum. In still others (birds, 

 some mammals) it passes insensibly into the flagellum, and no 

 sharply marked limit between them can be seen. In many of the 

 mammals the long connecting-piece is separated from the head by 

 a narrow "neck" in which the end-knobs lie, as described below. 



Internally, the middle-piece consists of an axial filament and an 

 envelope, both of which are continuous with those of the flagellum. 

 In some cases the envelope shows a distinctly spiral structure, like 

 that of the tail-envelope ; but this is not always visible. The most 

 interesting part of the. middle-piece is the "end-knob" in which the 

 axial filament terminates, at the base of the nucleus. In some cases 

 this appears to be single. More commonly it consists of two minute 

 bodies lying side by side (Fig. 50, B, D). This body is the only 

 structure in the middle-piece having the appearance of a centrosome ; 

 and Hermann conjectures that this is probably its real nature. 



The flagellum or tail is merely a locomotor organ which plays 

 no part in fertilization. It is, however, the most complex part of 

 the spermatozoon, and shows a very great diversity in structure. 

 Its most characteristic feature is the axial filament, which, as Bal- 

 lowitz has shown, is composed of a large number of parallel fibrillae, 

 like a muscle-fibre. This is surrounded by a cytoplasmic envelope, 

 which sometimes shows a striated or spiral structure, and in which, 

 or in connection with which, may be developed secondary or acces- 



