228 SUBNOTOCIIORDAL ROD. 



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

 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 PI. X. figs. 1 

 and la. The wall of the alimentary canal becomes thickened 

 (PI. X. fig. 1) along the median dorsal line, or else produced 

 into a ridge into which there penetrates a narrow prolonga- 

 tion of the lumen of the alimentary canal. In either case the 

 cells at the extreme summit of the thickening become gradu- 

 ally 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 w^all is very thick and undergoes no special 

 growth prior to the formation of the subnotochordal rod ; 

 on the contrary, a small linear portion of the wall becomes 

 scooped out along the median dorsal line, and eventually sepa- 

 rates 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 anterior 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 xiv. fig. 4:acc), 



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 my oldest Torpedo embryo (just younger than K) 

 there is no trace of it present. In a Torpedo embryo of 

 stage I the subnotochordal rod of the trunk terminated 

 anteriorly by uniting with the wall of the throat. The junction 

 was effected by a narrow pedicle, so that tlic rod appeared 



