174 THE PHYSIOLOGY OF REPRODUCTION 



d~ 



spermatozoon has tie essential characteristics of a complete cell. 

 The head contains the nuclear material, which is surrounded by 

 a thin layer of cytoplasm. The end-knob is said to represent 

 the centrosome. 



Spermatozoa, conforming with more or less closeness to the 

 type described above, occur in the greater majority of multi- 

 cellular animals from the Coelenterata up to Mammals. In 

 J 2 Pisces, and also in Echinoderms, the 



general resemblance is very distinct, but 

 in other forms of hfe there is more 

 diversity in the form assumed by the 

 spermatozoa. " 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 or a posterior piece of 

 different staining capacity, as is the 

 case with many birds and Mammals. 

 The achrosome sometimes appears to 

 be wanting, e.g. in some fishes. When 

 present, it is sometimes a minute 

 rounded knob, sometimes a sharp stylet, 

 and in some cases terminates in a sharp 

 barb-spur by which the spermatozoon ap- 

 pears to penetrate the ovum (Triton)." ^ 

 The middle-piece also shows considerable 

 variability. It may be spherical, cyUn- 

 drical, or flattened against the nucleus ; 

 sometimes it is of great length, and 

 sometimes it passes insensibly into the flageUum or tail. The 

 latter, in some insects and fishes, gives attachment to a mem- 

 branous fin. The end of the axial filament, as already mentioned, 

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



The tadpole-hke shape is not an essential characteristic of 



the spermatozoon, for in certain Arthropods and Nematodes 



there is no flageUum, and the sperms are consequently incapable 



of spontaneous movement. In the daphnid Polyphemus the 



^ Wilson, loc. cit. 



Tig. 47. — Human sperma- 

 tozoa ( X 1000). (After 

 Retzius, from Scha- 

 fer.) 



1, in profile ; 2, view on 

 the flat ; i, head ; c, 

 middle-piece ; d, tail ; 

 e, end-piece of tail, 

 described as a dis- 

 tinct part by Retzius. 



