36 THE FIRST PRINCIPLES OF HEREDITY 



(c) Spermatozoon. 



The Spermatozoon is, as a rule, very much smaller than 

 the corresponding ovum. Its form is adapted to its 

 function, for it actively seeks and penetrates the ovum. 



The typical spermatozoon — as, e.g., in man — consists of 

 a small pointed head, composed nearly entirely of the 

 nucleus, a middle piece, containing the centrosome, and a 

 long contractile tail, by means of which the spermatozoon 



Fig. 26. — Spermatozoa. 



(From Geddes and Thomson, " The Evolution of Sex.") 



a, crayfish ; b, lobster ; c, crab ; d, ascarid ; e, water-flea — moina ; 

 f, man ; §■, ray ; h, rat ; i, guinea-pig ; k, beetle — immature 

 stage ; /, sponge. 



effects its rapid undulatory movements. The shape in 

 other cases may differ, be starlike or clublike, but the 

 active amoeboid movements are an essential feature of it. 



III.— MA TURA TION. 



Both germ-cells, ovum as well as spermatozoon, possess 

 the same number of chromosomes as the ordinary body- 

 cells, which, as has already been remarked, is constant for 



