Fig. 413. Transverse section through the trunk of an embryo slightly older than fig. 28 E.
nc. neural canal; pr. posterior root of spinal nerve; x. subnotochordal rod; ao. aorta; sc. somatic mesoblast; sp. splanchnic mesoblast; mp. muscle-plate; mp´. portion of muscle-plate converted into muscle; Vv. portion of the vertebral plate which will give rise to the vertebral bodies; al. alimentary tract.
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 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.
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 terminates 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 alimentary tract, though it does not reach quite so far back as the caudal vesicle ([fig. 424], b x). Very shortly after it has attained its maximum size it begins to atrophy in front. We may therefore conclude that its atrophy, like its development, takes place from before backwards. During the later embryonic stages not a trace of it is to be seen. It has also been met with in Acipenser, Lepidosteus, the Teleostei, Petromyzon, and the Amphibia, in all of which it appears to develop in fundamentally the same way as in Elasmobranchii. In Acipenser it appears to persist in the adult as the subvertebral ligament (Bridge, Salensky). It has not yet been found in a fully developed form in any amniotic Vertebrate, though a thickening of the hypoblast, which may perhaps be a rudiment of it, has been found by Marshall and myself in the Chick ([fig. 110], x).
Eisig has instituted an interesting comparison between it and an organ which he has found in a family of Chætopods, the Capitellidæ. In these forms there is a tube underlying the alimentary tract for nearly its whole length, and opening into it in front, and probably behind. A remnant of such a tube might easily form a rudiment like the subnotochordal rod of the Ichthyopsida, and as Eisig points out the prolongation into the latter during its formation of the lumen of the alimentary tract distinctly favours such a view of its original nature. We can however hardly suppose that there is any direct genetic connection between Eisig’s organ in the Capitellidæ and the subnotochordal rod of the Chordata.
Splanchnic mesoblast and mesentery. The mesenteron consists at first of a simple hypoblastic tube, which however becomes enveloped by a layer of splanchnic mesoblast. This layer, which is not at first continued over the dorsal side of the mesenteron, gradually grows in, and interposes itself between the hypoblast of the mesenteron, and the organs above. At the same time it becomes differentiated into two layers, viz. an outer epithelioid layer which gives rise to part of the peritoneal epithelium, and an inner layer of undifferentiated cells which in time becomes converted into the connective tissue and muscular walls of the mesenteron. The connective tissue layers become first formed, while of the muscular layers the circular is the first to make its appearance.
Coincidently with their differentiation the connective tissue stratum of the peritoneum becomes established.