756 MESENTERY. 



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, a-). 



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. no, .v). 



Eisig has instituted an interesting comparison between it and an organ 

 which he has found in a family of Chretopods, the Capitellidse. 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 Capitellidse and 

 the subnotochordal rod of the Chordata. 



Splanchnic mesoblast and mesentery. The mescnteron 

 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 

 cpithelioid 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. 



