.\-i SPERMATOGENESIS 549 



p. 197). We have now to consider in greater detail Avhat is 

 knoAvn as to the precise mode of 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 of the gonad 

 is determined. 



In the spermary the sex-cells (Fig. 139, 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. Wti 

 saw (p. 544) that the number of chromosomes is constant 

 in 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. 139, the 

 ordinary cells of the body, including the primitive sex-cells, 

 contain twelve chromosomes, while the sperm-mothcr-cells 

 contain twenty-four. 



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

 chromosomes splitting in the ordinary way (p. 544, Fig 136) 

 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. The four cells 

 thus produced are the immature sperms (e) : in the majority 

 of cases the protoplasm of each undergoes a great elonga- 

 tion, being converted into a long vibratile thread, the tail oi 



