SPERMATOGENESIS. 



373 



lar cells. The result is that the nuclei of the latter are forced more 

 or less toward the wall of the seminal tubule, while their proto- 

 plasm is so indented by the adjacent spermatogonia that the cells 

 assume a flattened cylindric shape presenting indentations and 

 processes on all sides. In this stage the spermatogonia have a 

 radiate arrangement and entirely surround the elongated susten- 

 tacular cells. At present three periods are distinguished in the 

 development of the male sexual cells (spermatosomes) from the 

 spermatogonia. The first period embraces a repeated mitotic divi- 

 sion of the spermatogonia the period of proliferation. In the sec- 

 ond, the spermatogonia, which have naturally become smaller from 

 repeated division, begin to increase in size the period of growth. 

 The third is characterized by a modified double mitotic division 

 without intervening period of rest, and results in the matured sper- 

 matozoa the period of maturation, figure 300. During the third 

 period, a very important and significant process takes place the 



Primordial sexual cell. 



Spermatogonia. ,, 



\ 

 \ 



Spermatocyte I order. 



Spermatocytes II o 

 Sperma 



Zone of proliferation. 



(The generations are 



much larger.) 



Zone of growth. 



Zone of maturation. 



Fig. 300. Schematic diagram of spermatogenesis as it occurs in ascaris (after Boveri). 

 ("Ergebn. d. Anat. u. Entw.," Bd. I.) 



reduction in the number of chromosomes, so that in the spermatids, 

 the chromosomes are reduced to half the number present in a 

 somatic cell of the same animal. The manner in which this reduc- 

 tion in the number of chromosomes takes place will be described as 

 it occurs in salamandra maculosa. 



After the cells composing the last generation of spermato- 

 gonia have attained a certain size (period of growth), they under- 

 go karyokinetic division. First, the usual skein or spirem is 

 formed, but instead of dividing into twenty-four chromosomes, as 

 in the somatic cell, the filament of the skein segments into only 

 twelve loops. The cell thus provided with twelve chromosomes 

 now enters upon the period of maturation, and is known as a 



