and their Relation to the Peritoneum. 



475 



much, as they lead up to the points towards which it is desired to 

 draw attention. 



A section through the abdomen at about this period (the third or 

 fourth day) displays a body- wall enclosing a pleuro-peritoneal cavity ; 

 and in the interior of this an alimentary tube suspended by a 

 mesentery. A more minute examination of the mesentery shows that 

 it is attached to the spine above : that its surface is covered with 

 flattened cells, and that its interior consists of mesoblastic tissue and 

 blood-vessels. The latter spring from a large artery, the aorta, which 

 runs along the base of the serous fold. The vessels of the gut are not 

 merely derived from one or two branches arising from particular parts 

 of the aorta, but are represented by vessels which descend at intervals 

 from the parent trunk. Later it will be evident that only a few of 

 these mesenteric vessels persist, and that the various parts of the 

 alimentary canal retain a supply commensurate with- their require- 

 ments. Moreover, it will be seen that the development of a great 

 organ, such as the liver, pancreas, or spleen, in intimate relation with 

 the bowel, has an important influence in determining the size of the 

 vessels which pass through the mesentery. 



Again, referring to the section through the third or fourth day 

 chick, the following points are easily ascertained. The mesentery and 

 intestine hanging into the pleuro-peritoneal cavity divide it, roughly 

 speaking, into two halves ; a right and left (Diagram 1). Of course it 

 follows that the blood-vessels which it contains are bounded on each 

 side by these halves of the peritoneal cavity. As long as this arrange- 

 ment continues, nothing could be simpler than the relations of the 

 vessels ; but complications soon occur, and it will now be attempted to 

 state in detail what they are and how they are produced. 



For the sake of clearness, the events which take place in that part 

 of the mid-gut which afterwards becomes the stomach may be con- 



Diageam 1. 









(1 



jy[G ^ 











S. Stomach. MG. Mesogastrium. 



