38 



REPRODUCTIVE SYSTEM OF THE ADULT FROG 



maturation from the spermatogonium to the spermatozoon. The 

 spermatogonia are always located around the periphery of the semi- 

 niferous tubule and are small, closely packed cells, each with a 

 granular, oval nucleus. In between the spermatogonia may be found 

 occasional very large cells, the primary spermatocytes. These tend to 

 be irregularly spherical, possessing large and vesicular nuclei. The 

 cells are so large that they may be seen under low power magnifica- 

 tion (X 100) of the microscope. Apparently they divide to form 

 secondary spermatocytes almost immediately, for they are so few 

 and far between. The secondary spermatocytes (which develop as the 

 result of the first division) are about half the size of the primaries, 

 and lie toward the lumen of the tubule. They generally have a darkly 

 staining nucleus, and the cytoplasm may be tapered toward one 

 side. The spermatid, following another division, is even smaller and 

 possesses a condensed nucleus of irregular shape. Clusters of sperma- 

 tids appear as clusters of granules, the dark nucleus being almost as 

 small as the cross section of a sperm head. The metamorphic stages 

 from spermatid to spermatozoon are difficult to identify with ordinary 

 magnification, and are often confused with the spermatids themselves. 

 During this change the inner of two spermatid centrioles passes into 

 the nucleus while the outer one gives rise to the 

 tail-like flagellum. 



The mature spermatozoon averages about 0.03 

 mm. in length. It has an elongated, solid-staining 

 head (nucleus) with an anterior acrosome, point- 

 ing outwardly toward the periphery of the semi- 

 niferous tubule. The short middle piece generally 

 is not visible but the tail appears as a gray fila- 

 mentous extension into the lumen, about four or 

 more times the length of the sperm head. 



In any cross section of the testis, bundles of 

 sperm heads or tails may be cut at right angles 

 or tangentially, giving misleading suggestions of 

 structure. The mature spermatozoon is dependent 

 upon external sources of nutrition so that it joins 

 from 25 to 40 other spermatozoa, all of whose heads may be seen 

 converging into the cytoplasm of a relatively large, columnar-type 

 basal cell known as the Sertoli cell. This is functionally a nurse cell, 

 supplying nutriment to the clusters of mature spermatozoa until such 



Frog spermato- 

 zoon. Total length 

 0.03 to 0.04 mm. 



