DEVELOPMENT OF ELASMOBRANCH FISHES. 229 



mushroom- shaped in section, the stalk representing the pedicle 

 of attachment. 



On the formation of the dorsal aorta, the subnotochordal 

 rod becomes separated from the wall of the gut and the 

 aorta interposed between the two. 



The subnotochordal rod attains its fullest development 

 during stage K. Anteriorly it terminates at a point well in 

 front of the ear, though a little behind the end of the 

 notochord ; posteriorly it extends very nearly to the extremity 

 of the tail and is almost co-extensive with the postanal 

 section of the alimentary tract, though it does not quite 

 reach so far back as the caudal vesicle (PL xvii. fig. Qbx). In 

 stage L it is still fairly large in the tail, though it has begun 

 to atrophy anteriorly. We may therefore conclude that its 

 atrophy, like its development, takes place from before back- 

 wards. In the succeeding stages I have failed to find an}' trace 

 of it, and conclude, as does Professor Semper, that it dis- 

 appears completely. 



Gotte^ is of opinion that the subnotochordal rod is converted 

 into the dorsal lymphatic trunk, and regards it as the anterior 

 continuation of the postanal gut, which he believes to be also 

 converted into a hnnphatic trunk. My observations afford no 

 support to these views, and the fact already mentioned, that 

 the subnotochordal rod is nearly co-extensive with the postanal 

 section of the gut, renders it improbable that both these struc- 

 tures are connected with the lymphatic system. 



^ EntwicUungsgeschichte d. TJnke, p. 775. 



