SPERMATOGENESIS OF THE PIG. II 



numerous mitochrondia, will be reserved for a separate paper. 

 The germinal cells themselves are found in a great number of 

 seminiferous tubules coiled throughout the interior of the testes. 

 Sections of the tubules are found in groups of fifteen to twenty- 

 five which are completely surrounded by walls of connective 

 tissue. This connective tissue forms a sort of continuous net- 

 work of walls throughout the testes and is rather thick in places, 

 usually where three or four of the groups become appressed. 

 Embedded in these walls are blood- and lymph-vessels which 

 branch out into the masses of interstitial cells between the 

 tubules. 



The arrangement of the cells in the tubules is similar to that 

 of the other warm-blooded animals and the usual four types of 

 germinal cells are present. The spermatogonia form a more or 

 less regular layer of cells lying next to the wall of the tubule. 



These divide and give rise to two new cells, one or both of which 

 may become the primary spermatocyte. The primary spermato- 

 cytes are very abundant, especially in the spireme stage. After 

 various changes and considerable growth the primary spermato- 

 cytes divide and give rise to the secondary spermatocytes. The 

 secondary spermatocytes divide in turn to form the spermatids, 

 which transform directly into spermatozoa (Figs. 64-78). 



The material studied must have been in maximum activity at 

 the time of fixation, for the four types of germinal cells were 

 easily found in stages of growth and development. Mitotic 

 stages of spermatogonia, primary and secondary spermatocytes, 

 were very abundant, and frequently the entire field under the 

 oil immersion lens was composed of cells in mitosis. 



Spermatozoa in all stages of development were very abundant, 

 and clusters of the cells which were in the final stages of develop- 

 ment could be seen attached to the long cylindrical Sertoli cells 

 which often extend to the lumen of the tubule (Figs. 2 and 3). 

 The Sertoli cells, which are quite abundant, are closely connected 

 with the germinal cells and will be described in more detail later 



(Fig. 63). 



SPERMATOGONIA. 



The spermatogonia usually lie in a single layer next to the 

 wall of the tubule though occasionally some of the cells are 



