568 SPERMATOGENESIS CHAP. 



together in pairs, and the two members of each pair 

 undergo longitudinal division, so that a group of four 

 (tetrad) is formed from each pair. Thus, supposing the 

 number of chromosomes in the ordinary cells of the 

 body, including the primitive sex-cells, to be twelve 

 (Fig. 150, A), there will be six groups, each containing 

 four chromosomes, i.e., twenty-four in all, or double 

 the number characteristic of the species (B) . As the 

 spindle is formed, these tetrads become arranged on its 

 equator, two chromosomes from each group then passing 

 to one pole and two to the other. The cell now divides, 

 and the nucleus of each daughter-cell thus contains 

 twelve chromosomes (C). The process of division, with- 

 out a return to the spireme stage (p. 560), is then repeated 

 in each ; but the chromosomes this time do not split 

 longitudinally, six of them passing entire to each pole 

 of the spindle respectively. Cell-division follows, and 

 it will be seen that each of the four cells derived from the 

 original mother-cells contains only six chromosomes (D), 

 i.e., half the normal number, and the process is there- 

 fore known as a reducing division. 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 constitutes 

 its more or less spindle-shaped head and the centrosome 

 is included in a small middle piece at the junction of head 

 and tail (Fig. 148). 



Thus the sperm or male gamete is a true cell specially 

 modified in most cases for active movement. This 

 actively motile, tailed form is, however, by no means 

 essential : in some animals (e.g. crayfish, p. 382) the 

 sperms are non-motile and of peculiar form. 



The ova arise from primitive sex-cells, precisely 

 resembling those which give rise to sperms. They 



