MESENTERON. 



621 



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

 notochord (fig. 413, x). 



In the hindermost 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- 

 notochordal 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 splitting off 

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

 it is formed before the posterior. 



FIG. 412. TIIANSVEKSE SECTION 

 THROUGH THE TAIL EEGION OF A PKISTI- 

 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 splanchnic 

 mesoblast commencing to be differen- 

 tiated into muscles ; ch. notochord; x. 

 snbnotochordal rod arising as an out- 

 growth of the dorsal wall of the ali- 

 mentary tract; al. alimentary tract. 



FIG. 413. TRANSVERSE SEC- 

 TION THROUGH THE TRUNK OF AN 

 EMKRYO SLIGHTLY OLDER THAN 

 FIG. 28 E. 



nc. neural canal ; pr. posterior 

 root of spinal nerve; .r. subnoto- 

 chordal rod; ao. aorta; sc. somatic 

 mesoblast; sj). splanchnic meso- 

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

 tion of muscle-plate converted 

 into muscle; I'r. 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 becomes 

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

 two (fig. 367, x). 



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 notoehord ; posteriorly it extends very nearly 

 to the extremity of the tail and is almost co-extensloe ivith the pustanal 

 section of the alimentary tract, though it does not reach quite so far back as 



