THE SPERJ/ATOZOdN 



139 



the spermatozoon of Myzostonia, where it is a greatly elono"ated 

 fusiform body, passing insensibly into the tail without distinct middle- 

 piece and containing a single series of chromatin-discs. The num- 

 ber of these in J/, glabrnm is 24, which is the somatic number 

 of chromosomes in this species. In AT. cirrifcnivi the number of 

 chromatin-discs is more than 60. Somewhat similar spermatozoa 

 occur in the acoelous Turbellaria.^ The acrosome sometimes appears 

 to be wanting, e.g. in some fishes (Fig. 66). 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 sperma- 

 tozoon appears to penetrate the ovum {Triton). In the mammals it 

 is sometimes very small (rat), sometimes very large (guinea-pig), and 

 in some forms is surrounded by a cytoplasmic layer forming the 

 "head-cap" (Figs. 68, 86). 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. 

 66-68). 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 distinguishable 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 or more 

 minute bodies lying side by side (Fig. 68, B, D). 



The flagellum or tail is merely a locomotor organ which plays no 

 part in fertihzation. 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 filavicnt, which, as Ballowitz 

 has shown, is composed of a large number of parallel fibrillse, like a 

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

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



1 Cf. Wheeler, p. 7. 



