56 Jeffrey Pudney, Jacob A. Canick, Gloria V. Callard 



resulting in a spatial and temporal segregation of differentiating germ 

 cells. Depending on the region and presumably regulated by the temper- 

 ature of the environment, for Necturus this wave of spermatogenesis can 

 begin in the spring or early summer and is concluded by late summer or 

 fall. In many urodeles this wave of spermatogenesis is slow. Due to the 

 tardy differentiation of germ cells in these species the testis becomes 

 divided into lobes that contain germinal elements at different stages of 

 development. In Necturus, however, the spermatogenic wave is compar- 

 atively rapid and thus lobation of the testis does not occur. Further- 

 more, the process of spermatogenesis is restricted to those regions of the 

 seminiferous lobules distal to the central collecting duct, while lobules 

 proximal to this duct contain groups of undifferentiated germ cells, the 

 spermatogonia (see Fig. 2). These germ cells are in a "resting stage," 

 providing a reservoir for successive waves of spermatogenesis. Thus, in 

 N. lewisi there is a maturational wave of germ cells occurring longitudi- 

 nally in a caudocephalic direction, plus a proximal to distal cycle of 

 differentiating germ cells to form the maturing segments of the seminif- 

 erous lobules that initiate seasonal recrudescence (Fig. 2). Cyst devel- 

 opment begins when a single large cell, the primary spermatogonium, 

 becomes associated with several somatic cells. Mitotic divisions of these 

 primary spermatogonia results in the formation of cysts (enclosed by the 

 seminiferous lobule), containing secondary spermatogonia. Subsequent 

 maturational divisons of these spermatogonia cause an increase in the 

 size of the cysts by the development of spermatocytes that differentiate 

 into spermatids. During spermiogenesis the developing spermatozoa are 

 still enclosed by somatic cells and the integrity of the cysts remains 

 intact. By the time the final stages of spermatozoan maturation are 

 reached, however, the cysts are highly distended. Presumably due to this 

 large increase in size the cysts now rupture, resulting in the spermatozoa 

 (embedded in somatic cells) lying free within the confines of the seminif- 

 erous lobule. The release of spermatozoa from the somatic cells, by the 

 act of spermiation, now allows exit of the spermatozoa via the lumen of 

 the lobules into the main collecting duct. It should be emphasized that 

 once the cysts enter into the spermatogenetic process the germ cells 

 present in each of these structures undergo the various developmental 

 changes synchronously in a given seminiferous lobule. Thus, each lobule 

 will, in successive periods, contain in its maturing portion, spermato- 

 cytes, spermatids, and finally spermatozoa. The maturing portion will 

 never at any one time contain all, or a combination of, these stages as 

 occurs in the seminiferous tubules of the amniotes. 



It has been previously demonstrated that in Necturus maculosus 

 the longitudinal wave of spermatogenesis is also reflected in the degree 

 of development of the adjacent interlobular tissue (Humphrey 1921; 

 Pudney et al. 1983). The same relationship also prevails in the testis of 



