THE MALE 



35 



During these changes in the chromatin material of the nucleus, 

 the volume of the nucleus and the cytoplasm are considerably in- 

 creased, the nuclear membrane breaks down, and the chromosomes 

 assume bizarre shapes and various sizes. They may be paired, curved, 

 or straight; "V" and "C" and reversed "L" shapes, figure 8's, and 

 grouped as tetrads. This is known as the diakinesis stage. The chromo- 

 somes are then lined up on a spindle in anticipation of the first of 

 the two maturation divisions. 



Spermatogenesis in the frog is seasonal and is completed within 

 the testes. The walls of the seminiferous tubules produce spermato- 

 gonia which go through mitotic divisions and then the series of 

 nuclear changes (described above) without mitosis. This results in 

 the appearance, toward the lumen of each tubule, of clusters of ma- 

 ture spermatozoa. By the time of hibernation (October) all the 

 spermatozoa that are to become available for the following spring 

 breeding season will have matured. At this time the testis will ex- 

 hibit only these spermatozoa and relatively few spermatogonia, with- 

 out the intervening maturation stages. The spermatogonia are found 

 close to the basement membrane of the seminiferous tubule. These 

 then await their turn to undergo the maturation changes necessary 

 for the production of spermatozoa which will be ready for the breed- 

 ing season a year and a half thereafter. The elongated and filamentous 



April 



May June July August September October November Dec- Mor. 



INTERSTITIAL 

 TISSUE 



THUMB PAD z" 



MATURATION 



Normal cyclic changes in the primary and secondary sexual characters of the 

 frog, Rana pipiens. (From Glass and Rugh, 1944, J. MorphoL, 74:409.) 



