566 SPERMATOGENESIS CHAP. 



development of sperms (spermatogenesis] and of ova 

 (oogenesis), as well as the exact steps of the process by 

 which an oosperm or unicellular embryo is formed by the 

 union of the two sexual elements. 



Both ovary and spermary are at first composed of cells of 

 the ordinary kind, the primitive sex-cells ; and it is only by 

 the further development of these that the sex is determined. 



In the spermary the sex-cells (Fig. 149, A) undergo re- 

 peated fission, forming what are known as the sperm-mother- 

 cells (B). These have been found in several instances to be 

 distinguished by a peculiar condition of the nucleus. We 

 saw (p. 561) that the number of chromosomes is constant 

 in any given animal, though varying greatly in different 

 species. At an earlier stage 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. 149, twelve chromosomes can be 

 recognised in the ordinary cells of the body, including the 

 primitive sex-cells, while 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. 561, Fig. 146) 

 half of their total number in the present instance twelve 

 passes into each daughter cell : thus two cells are pro- 

 duced having the normal number of chromosomes. The 

 process of division is immediately repeated in the same pecu- 

 liar 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: this is there- 

 fore 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 



