XII 



SPERMATOGEXESIS 



567 



In these, as well as in the egg-mother-cells (p. 569), the 

 nucleus, instead of dividing in the ordinary way, as 

 described on p. 560, undergoes a special kind of division. 



FIG. 150. Spennatogenesis in the Mole-cricket (Gryllotalpa). 

 A, primitive sex-cell, just preparatory to division, showing twelve chromosomes 

 arranged in tetrads (ckr) c. the centrosome. B, sperm-mother-cell, formed by 

 the division of A, and containing twenty-four chromosomes arranged in tetrads ; 

 the centrosome has divided into two. C, the sperm-mother-cell has divided 

 into two, each daughter-cell containing twelve chromosomes. D, each daughter- 

 cell has divided again, a group of four sperm-cells being produced, each with 

 only six chromosomes. E, a single sperm-cell about to elongate to form a 

 sperm. F, immature sperm ; the six chromosomes are still visible in the head. 

 G, hilly-formed sperm. (From Parker's Biology, after vom Rath.) 



We saw that the number of chromosomes is in general 

 constant in any given animal, though varying greatly 

 in different species. Xow in the nucleus of the sperm- 

 mother-cells the chromosomes become applied close 



