THE MULTIPLE TESTIS OF URODELES. 53 



in which, during the first season of the male's sexual activity, the 

 germ cells did not develop beyond the secondary spermatogonial 

 stage. In this way the testis is increased in length somewhat from 

 year to year as the animal develops. 



In the Dcsinoffnathiis male, however, the extrusion of the sper- 

 matozoa in autumn is not followed, as in Plcthodon, by prompt re- 

 generation of the emptied lobules. The few residual spermato- 

 gonia located at the apex of each lobule appear to remain quiescent 

 for several months. During the winter the emptied lobules slowly 

 degenerate. The connective tissue cells surrounding them hyper- 

 trophy, forming interstitial cells, as described in detail in a previous 

 communication. In the following summer the testis appears as in 

 Fig. 2 of Chart I. The region anterior to the boundary plane b 

 in Fig. i has developed to form the functional testis, in which the 

 forward movement of the spermatogenetic wave proceeds slowly 

 until again checked at the new boundary plane b'. The region 

 containing degenerating lobules and interstitial cells is shown pos- 

 terior to b (cross-hatched). It is to be kept in mind, of course, 

 that the longitudinal collecting duct, surrounded by scattered 

 groups of residual spermatogonia, extends throughout this region. 

 The degenerating lobules and interstitial cells gradually disappear, 

 until finally only the collecting duct and residual spermatogonia 

 remain. In Fig. 2.A the testis is represented as seen late in the 

 summer following the animal's first year of sexual activity. The 

 extreme caudal part, reduced to a condition similar to that of the 

 anterior cord of spermatogonia, may be referred to as the " caudal 

 germ-cell cord ". The functioning region of the testis shown in 

 Fig. 2.A empties, and the cycle of changes outlined above is re- 

 peated. The caudal germ-cell cord, in this way, is increased in 

 length, as shown in Fig. 3. This figure represents the testis of a 

 male as in the summer after the second extrusion of spermatozoa. 

 The positions of the boundary planes of successive seasons are 

 indicated by b, b f , and b". 



Finally, after several months of inactivity, the residual sperma- 

 togonia in the most posterior part of the caudal germ-cell cord 

 begin to multiply. Their multiplication leads to the formation oi 

 lobules of secondary spermatogonia ; these in due season become 

 spermatocytes I. and finally mature as spermatozoa. The testis, 



