168 THE PHYSIOLOGY OF REPRODUCTION 



to be wanting .//. in some fishes. When present, it is sometimes a 

 minute rounded knob, sometimes a sharp stylet, and in some cases 

 t.M-minates in a sharp barb-spur by which the spermatozoon appears 

 to penetrate the ovum (Triton)."* The middle-piece also shows 

 considerable variability. It may be spherical, cylindrical, or flattened 

 against the nucleus ; sometimes it is of great length, and sometimes 

 it passes insensibly into the flagellum or tail. The latter, in 

 s .:ue insects and fishes, gives attachment to a membranous fin. 

 The end of the axial filament, as already mentioned, 

 A /fc is sometimes left naked, giving rise to the end-piece. 

 ^ ' The tadpole-like shape is not an essential char- 



acteristic of the spermatozoon, for in certain Arthro- 

 pods and Nematodes there is no flagellum, and the 

 sperms are consequently incapable of spontaneous 

 movement. In the daphnid Polyphemus the sperms 

 are said to be amoeboid. In some crustacean sper- 

 matozoa there are a number of radiating spine-like 

 processes which seem to take the place of the 

 flagellum. 



In other animals, and notably in the gasteropod 

 mollusc /'nfinliiin, there are two kinds of sper- 

 matozoa. In this animal one is of the usual type, 

 whereas the other is larger and worm-shaped, with a 

 tuft of cilia at one end. The smaller variety alone 



i 



is said to be functional.- 



The s j ze O f the S p e rm varies greatly in different 

 animals. In man its length is about -05 millimetre 



Or a S00th f an inch) the head and the middle - 

 flat ; I, head ; piece being each about '005 millimetre long. 



, middle-piece; j t j s obvious that the sperm contributes com- 

 '/, tail ; e. end- . , , , P , .,. , 



piece of tail, paratively little material to the fertilised ovum, 

 described as a being provided with only sufficient protoplasmic 

 substance to form a locomotive apparatus by means 

 of which it gains access to the ovum. The pre- 

 dominantly destructive metabolism of the spermatozoon as con- 

 trasted with the ovum has been strongly emphasised by Geddes 



1 Wilson, /'. <!(. 



2 For further details of tin- structure of various kinds of sperms see Wilson, 

 />: fit. ; alao Ballowit/.'s papers just referred to, and Retzius' Bioloqisclie 



:-*nchiii>ii>-ii. v "' s - xi- xii-. and xiii., Stockholm and Jena. The latter 

 contains numerou- lar^c plates with figures of spermatozoa. For sexual 

 differences in sjM-i-ms (numbers of chromosomes, etc.), see Chapter XIII. See 

 also von Winiwai t<T, . I /</,. ,/, /;/<,/., vol. xxvii., 191-2 (man) ; Wodsedalek, Ilial. 

 /!"'/., vol. xxv., !!)]:{ (pit:): vol. xxvii., 1914 (horse); Leplat, Arch, de /iinf., 

 vol. xxv., 191O (<-at; ; 1'ainter. ./,,,/,-. /-.'.,/,. /<,<,/., vol. xxxv., 1922 (opossum); 

 (Iiiyei, Ilinl. /!"ff., vol. .\xi., U)lfi (fowl); and Swift, Amer. Jour, of An.at., 

 vol. xx., 19H5 (fowl). Sc. also references on p. 128 above. 



Human sperma- 



tozoa ( x 1000). 

 (After Retzius, 



* n) 



