622 MESENTERY. 



the caudal vesicle (fig. 424, hx). 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 AmphiV)ia, in all of which it appears to develop in funda- 

 mentally the same way as in Elasmobranchii. In Acipenser it appears to 

 ]jersist in the adult as the subvertebral ligament (Bridge, Salensky). It 

 has not yet been found in a fully developed form in any amniotic Verte- 

 brate, 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, 4 



Eisig has instituted an interesting comparison between it and an 

 organ which he has found in a family of Chsetopods, the Capitellid*. In 

 these forms there is a tube underlying the alimentary tract for nearly its 

 whole length, and opening into it in front, and j>robab]y behind. A remnant 

 of such a tiibe 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 anv direct genetic connection between Eisig's organ in the 

 Capitellidse and the subnotochordal rod of the Chordata, 



Splanchnic mesoblast and mesentery. The mesenteron consists 

 at first of a simple hypoblastic tube, which however becomes enve- 

 loped 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 mesen- 

 teron, and the organs above. At the same time it becomes dif- 

 ferentiated 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 con- 

 nective 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. 



The Mesentery. Prior to the splanchnic mesoblast growing round 

 the alimentary tube above, the attachment of the latter structure to 

 the dorsal wall of the body is very wide. On the completion of this 

 investment the layer of mesoblast suspending the alimentary tract 

 becomes thinner, and at the same time the alimentary canal appears 

 to be drawn downwards and away from the vertebral column. 



In what may be regarded as the thoracic division of the general 

 pleuroperitoneal space, along that part of the alimentary canal which 

 will form the oesophagus, this withdrawal is very slight, but it is 

 very marked in the abdominal region. In the latter the at first 

 straight digestive canal comes to be suspended from the body above 

 by a narrow flattened band of mesoblastic tissue. This flattened 

 band is the mesentery, shewn commencing in fig. 117, and much 



