STEUCTURE OF THE SPERMATOZOA. 2)7 



Spermatogenesis. The spermatozoa are developed from the small cells 

 (spermatoblasts) which form the innermost stratum of the seminal epi- 

 thelium, and these are themselves produced by the division of the large 

 spermatogeiiic or mother-cells of the second layer. It is probable that these 

 mother-cells again are formed by division of some of the lining epithelium- 

 cells. The cycle of changes therefore which appears to take place is as 

 follows : 1. Division of a lining epithelium-cell into two, one of which 

 becomes a spermatogen, and passes into the second layer, while the other 

 remains in the first layer, undergoes enlargement, and becomes a susten- 

 tacular cell. 2. Division of the spermatogen. 3. Further division and 

 multiplication of the spermatogens and the conversion of the resulting 

 daughter-cells into a group of spermatoblasts. 4. Elongation of the sperma- 

 toblasts and their gradual conversion into mature spermatozoa. As they 



FIG. 241. DIAGRAM EXHIBITING THE CYCLE OF PHASES OF SPERMATOGENESIS (RAT). 



a, lining epithelium-cells, seen dividing in 6 ; a', sustentacular-cells ; 6, spermatogenic or 

 mother-cells, with skein-like nuclear filaments. These cells are seen actively dividing 

 in 5. c, spermatoblasts, forming an irregular column or clump in 6, 7, 8, and 1, and 

 connected to an enlarged supporting cell, a', of the lining epithelium in 2, 3, 4, and 5. 

 In 6, 7, and 8 advanced spermatozoa of one crop are seen between columns of sperma- 

 toblasts of the next crop, s', parts of the spermatoblasts which are disintegrated when 

 the spermatozoa are fully formed ; s, seminal granules resulting from their disintegra- 

 tion ; a" in 1 and 2 are nuclei of supporting cells which are probably becoming extruded. 



undergo this conversion their grouping becomes more evident, and each group 

 is found to be connected with a sustentacular cell, which probably ministers 

 to their nutrition. This cell undergoes a gradual process of elongation so 

 that the spermatozoa by the time they are fully developed are brought to 

 the lumen of the tube, in which they then become free. In the meantime 

 other alternate groups of daughter-cells from which the next crop of sperma- 

 tozoa will be derived are being formed in the same manner, passing through 

 the same cycle of changes. So that in a section of the same tubule, at least 



