THE MULTIPLE TESTIS OF URODELES. 55 



at the time the lobules first begin regeneration, appears as in Fig. 4. 

 A completely developed second lobe is represented in Fig. 5. 



The spermatogenetic wave moves slowly cephalad along the 

 caudal germ-cell cord in the same fashion as it moved along the 

 primary germ-cell cord in the case of the first lobe. Boundary 

 planes are established, in each season, in the same manner ; anterior 

 to these the transformation of the spermatogonia into spermato- 

 cytes I. ceases. In other words, a second " testis " has been estab- 

 lished, caudal to the first and separated from it by the so-called 

 sterile region a region sterile only in the sense that its germ cells 

 are temporarily inactive. This "testis" or lobe shifts forward on 

 the trail of the first, leaving behind it, when first it empties, a 

 caudal degenerating region such as is represented in Fig. 6. By 

 the disappearance of the degenerating lobules and interstitial cells, 



occupied by degenerating- lobules and interstitial cells. Fig. zA represents 

 the testis as seen in later summer; all other diagrams are as of testes in 

 June after the boundary plane (b, b', etc.) for the season's development of 

 spermatocytes has been established. 



FIGS. 15 TO 19. Diagrams of the cephalic portion of the testis of Des- 

 mognathus, illustrating the disappearance by " running out " of an anterior 

 lobe. Regions of the testis are represented as in the preceding figures. 

 Figs. 15 and 16, testes as in June. Fig. 17, the same testis as in Fig. 15 or 

 1 6, but in June one year later. Fig. 18, the same testis later in the summer, 

 prior to the final disappearance of the lobe. Fig. 19. the same testis in 

 September after the last trace of the lobe has disappeared. 



FIGS. 20 AND 21. Diagrams of multiple testes of Destnognatlius approach- 

 ing a simple type by reduction of the number of lobes. 



FIGS. 22 AND 23. Diagrams indicating the differences observed between 

 the testes of an individual. One testis (Fig. 23) is a season ahead of the 

 other in development, as indicated by the earlier disappearance of the an- 

 terior lobe and the earlier development of a new lobe caudally. 



FIG. 24. Testis of Plethodon male for comparison with Fig. i. Repre- 

 sented as in midsummer. Note the absence of a long anterior germ-cell cord. 



FIG. 25. The testis of Fig. 24 as in the fall, some time after extrusion 

 cf the spermatozoa had begun. Region A contains developing lobules of 

 secondary spermatogonia for the next sexual cycle. Region B contains less 

 well-developed lobules, and many degenerating lobules and interstitial cells. 

 In region C the lobules still contain spermatozoa, but the lobules of the 

 next cycle are already beginning their development. Black in this and in 

 Fig. 24 indicates the part of the testis not maturing spermatozoa in the 

 first season ; i.e., a region containing only primary and secondary spermato- 

 gonia. Fig. 25 illustrates the nearest approach to lobatioii in the testis of 

 Plethodon; the rapid development of lobules in regions B and C soon re- 

 stores the testis to the type of Fig. 24. 



