THE SUBNOTOCHORDAL ROD. 457 



of organs it ought to be treated. In the presence of this 

 difficulty it seemed best to deal with it in this chapter, since it 

 is unquestionably developed from the wall of the alimentary 

 canal. 



At its full growth this body forms a rod underlying the 

 notochord, and has nearly the same longitudinal extension as 

 this. It is indicated in most of my sections by the letter x. 

 We may distinguish two sections of it, the one situated in the 

 head, the other in the trunk. The junction between the two 

 occurs at the hind border of the visceral clefts. 



The section in the trunk is the first to develope. It arises 

 during stage H in the manner illustrated in PL n, figs, i and \a. 

 The wall of the alimentary canal becomes thickened (PI. 11, 

 fig. i) along the median dorsal line, or else produced into a 

 ridge into which there penetrates a narrow prolongation of the 

 lumen of the alimentary canal. In either case the cells at the 

 extreme summit of the thickening become gradually constricted 

 off as a rod, which lies immediately dorsal to the alimentary 

 tract, and ventral to the notochord. The shape of the rod 

 varies in the different regions of the body, but it is always 

 more or less elliptical in section. Owing to its small size and 

 soft structure it is easily distorted in the process of preparing 

 sections. 



In the hindermost part of the body its mode of formation 

 differs somewhat from that above described. In this part the 

 alimentary wall is very thick and undergoes no special growth 

 prior to the formation of the subnotochordal rod ; on the con- 

 trary, a small linear portion of the wall becomes scooped out 

 along the median dorsal line, and eventually separates from the 

 remainder as the rod in question. In the trunk the splitting off 

 of the rod takes place from before backwards, so that the an- 

 terior part of it is formed before the posterior. 



The section of the subnotochordal rod in the head would 

 appear from my observations on Pristiurus to develope in the 

 same way as in the trunk, and the splitting off from the throat 

 proceeds from before backwards (PL 15, fig. 4^ x). 



In Torpedo, this rod developes very much later in the head 

 than in the trunk ; and indeed my conclusion that it developes 

 in the head at all is only based on grounds of analogy, since in 



B. 30 



