xxiii REDUCING DIVISION 255 



any given animal, though varying greatly in different species. 

 In the formation of the sperm-mother-cells from the primitive 

 sex-cells the number becomes doubled : in the case of 

 the mole-cricket, for instance, shown in Fig. 61, while the 

 ordinary cells of the body, including the primitive sex- 

 cells, * contain twelves chromosomes, the sperm-mother-cells 

 contain twenty-four. 



The sperm-mother-cell now divides (c), but instead of its 

 chromosomes splitting in the ordinary way (p. 64 and Fig. 10) 

 half of their total number — in the present instance twelve — 

 passes into each daughter cell : in this way two cells are 

 produced having the normal number of chromosomes. The 

 process of division is immediately repeated in the same 

 peculiar way (d), the result being that each sperm-mother- 

 cell gives rise to a group of four cells having half the normal 

 number of chromosomes — in the present instance six. The 

 four cells thus produced are the immature sperms (e) : in 

 the majority of cases the protoplasm of each undergoes a 

 great elongation, being converted into a long vibratile thread, 

 the tail of the sperm (f, g), while the nucleus becomes its 

 more or less spindle-shaped head and the centrosome takes 

 the form of a small mtermediate piece at the junction of 

 head and tail. 



Thus the sperm or male gamete is a true cell, specially 

 modified in most cases for active movement : its head, 

 representing the nucleus, is directed forwards in progres- 

 sion, its long tail, formed from the protoplasm, backwards. 

 The direction of movement is thus the precise opposite of 

 that of a monad (p. 36) to which a sperm presents a certain 

 resemblance. This actively motile tailed form is, however, 

 by no means essential : in many animals the sperms are 

 non-motile and in some they resemble ordinary cells. 



The peculiar variety of mitosis described above, by which 



