34 WILLIAM MORTON WHEELER, 



a comparisou ot Ammocœtes of different stages aud the adult Petro- 

 myzon. Thus in the adult P. marinus there is a distance of at least 

 32 instead of 7 segments from the region of the pronephros to 

 the beginning of the mesonephros, the latter extending through 39 

 segments and terminating only a short distance in front of the anus. 

 Hence the mesonephros must have atrophied over a space of 25 seg- 

 ments, since there is no evidence that the anterior portion of the 

 organ has been crowded back. The study of stages between the larva 

 of 12 mm and the larva of 17 cm shows that this atrophy has gone 

 on pari passu with the addition of new tubules to the posterior ends 

 of the mesonephros. This is readily determined, because the meso- 

 nephric tubules, as soon as they are fully developed, cause the ori- 

 ginally very small nephric lobes (cf. Figs. 46 and 47) to increase 

 greatly in size and to assume the appearance of Fig. 65, so that they 

 are distinctly marked off in lateral view from the portions which 

 enclose only the pronephric duct and the undeveloped tubules. W e 

 are, therefore, justified in maintaining that the meso- 

 nephros of the young Ammocœtes is not the same organ 

 as the mesonephros of the old Ammocœtes or the Fetro- 

 myzan, the mesonephros in the former being as truly a 

 larval structure as the pronephros since its existence 

 and function are confined to larval life. 



Another fact also should be borne in mind throughout the fol- 

 lowing account, viz that there are no observable traces of 

 metamerism in the organ. Both glomeruh and tubules are 

 more numerous than the myotomes and spinal ganglia. The nephric 

 arterioles from the aorta are, on the contrary, less numerous than the 

 segments and often quite irregular, 



b) The Development of the Mesonephric Tubules. 



The formation of the very first tubules I have not seen. This 

 must occur in larvœ about 10 mm long. But by the time the larva 

 is 12 to 15 mm long, so few tubules are presenty that it is possible 

 from studying the posterior end of the organ to form an adequate 

 idea of the way in which the anterior end must have developed. 

 Figs. 63 and 64 represent cross-sections through the mesonephros of 

 a larva 12 mm long; the former passes through a region where the 

 tubules though still young are nevertheless long and somewhat contorted, 

 the latter passes through a region a little further back, where the 

 tubules are still in statu nascendi Confining our attention to this 



