44 WILLIAM MORTON WHEELER, 



side of the raid-ventral line of the intestine and it lias disappeared 

 before the section in which the median subcardinal sinus receives its 

 vessel. 



In the larva 7 cm long the aorta is found sending down vessels 

 alternately and at rather long and irregular intervals into the meso- 

 nephric lobes. These arteries leave the floor of the aorta at the in- 

 sertion of the subaortic septa, traverse the full length of these septa, 

 enter the corpus adiposum and pass down its mesial portion near the 

 peritoneum to supply the glomerulus. At first the vessels are rather 

 tortuous as if they had difficulty in making their way through tlie 

 trabecular tissue, but later they become nearly or quite straight. In 

 older larvaî and in the adult Fetromyzon marinus they have thick 

 muscular walls. When they reach the glomerulus they branch, sending 

 vessels anteriorly and posteriorly as indicated in Fig. 59 (gl a). The 

 aorta grows very small when it becomes the caudal artery, and in 

 the younger Ammocœtes examined (7 cm) I find no branches de- 

 scending from it through the vestigial mesentery to the posterior portion 

 of the intestine. 



Of previous writers on the development of the mesonephros only 

 FüRBRiNGER (1876) and Vialleton (1890) derive the tubules from 

 the peritoneal epithelium. Both, however, refer to very different 

 portions of the mesonephros, the former to the extreme anterior end 

 in very young Ammocœtes (9 mm), the latter to the posterior end in 

 much older larvae. Bujor (1891) who studied the development of the 

 tubules during metamorphosis, leaves the origin of the mesonephric 

 tubules undecided, since he finds the young cells of the corpus ad- 

 iposum very similar to the cells of the young tubules. This is to 

 some extent the case in young Ammocœtes, but in the large larva 

 (17 cm long) corresponding very nearly to Bujor's stages, the tubule- 

 cells are very unlike the cells of the fat body and closely resemble 

 those of the peritoneum. 



According to FtJRBRiNGER, the tubules of the mesonephros have 

 a metameric origin, but Vialleton as definitely claims that they are 

 not metameric but more numerous than the segments. Since, however, 

 they are writing of different portions of the mesonephros, both might 

 be correct, especially if their interpretations are judged in the light 

 of such Vertebrates as the Amphibia, where authors have found an 

 increase in the number of mesonephric tubules caudally , with a 



