MESENTERON. 



755 



It develops in Elasmobranch embryos in two sections, one situated in 

 the head, and the other in the trunk. 



The section in the trunk is the first to appear. The waU~of the 

 alimentary canal becomes thickened along the median dorsal line (fig. 412, 

 x), 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 become gradually constricted off as a rod, which lies 

 immediately dorsal to the alimentary tract, and ventral to the notochord 

 (fig. 413, x). 



t>r 



mp 



FIG. 412. TRANSVERSE SECTION 

 THROUGH THE TAIL REGION OF A 

 PRISTIURUS EMBRYO OF THE SAME 

 AGE AS FIG. 28 E. 



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

 //. 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. 

 subnotochordal 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 

 EMBRYO SLIGHTLY OLDER THAN 

 FIG. 18 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; /;//. por- 

 tion of muscle-plate converted into 

 muscle ; Vv. portion of the vertebral 

 plate which will give rise to the ver- 

 tebral bodies; al. alimentary tract. 



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. 



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. 



482 



