MESENTERON. 



621 



which lies immediately dorsal to the alimentary tract, and ventral to the 

 notocliord (fig, 413, x). 



In the hinderraost part of the body its mode of formation differs some- 

 what from that above described. In this part the alimentary wall is very 

 thick, and undergoes no special growth prior to the formation of the sub- 

 iiotochordal rod ; on the contrary, 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 sjditting off 

 of the rod takes place from before backwards, so that the anterior part of 

 it is formed before the posterior. 



Fig. 412. Transverse section 



THROUGH THE TAIL REGION OF A PrISTI- 

 URUS EMBRYO OF THE SAME AGE AS FIG. 



28 E. 



df. dorsal fin; sp.c. spinal cord; 

 pp. body cavity; sp. splanchnic layer 

 of mesoblast; so. somatic layer of 

 mesoblast; mp'. portion of splanclinic 

 mesoblast commencing to be differen- 

 tiated into muscles; ch. iiotochord; a:, 

 subnotochord&l rod arising as an out- 

 growth of the dorsal wall of the ali- 

 mentary tract; al. alimentary tract. 



mp 



Fig. 413. Transverse sec- 

 tion THROUGH THE TRUNK OF AN 



embrto slightly older than 

 riG. 28 E. 



nc^ neural canal ; pr. posterior 

 root of spinal nerve; x. subnoto- 

 chordal rod; ao. aorta; sc. somatic 

 mesoblast; sp. splanchnic meso- 

 blast ; mp. muscle-plate ; mj)'. por- 

 tion of muscle-plate converted 

 into muscle; Vi\ portion of the 

 vertebral plate which will give rise 

 to the vertebral bodies ; al. ali- 

 mentary tract. 



The section of the subnotochordal rod in the head would appear to 

 develop in the same way as that in the trunk, and the splitting off from 

 the throat proceeds from before backwards. 



On the formation of the dorsal aorta, the subnotochordal rod l^ecomes 

 separated from the wall of the gut and the aorta interposed between the 

 two (fig. 367, a;). 



When the subnotochordal rod attains its fullest development it ter- 

 minates anteriorly some way in front of the auditory vesicle, 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 alinientary tract, though it does not reach quite so far back as 



