758 MESENTERY. 



The mesentery of the stomach, or mesogastrium, enlarges in 

 Mammalia to form a peculiar sack known as the greater 

 omentum. 



The mesenteron exhibits very early a trifold division. An 

 anterior portion, extending as far as the stomach, becomes 

 separated off as the respiratory division. On the formation 

 of the anal invagination the portion of the mesenteron behind 

 the anus becomes marked off as the postanal division, and 

 between the postanal section and the respiratory division is a 

 middle portion forming an intestinal and cloacal division. 



The respiratory division of the mesenteron. 



This section of the alimentary canal is distinguished by the 

 fact that its walls send out a series of paired diverticula, which 

 meet the skin, and after a perforation has been effected at the 

 regions of contact, form the branchial or visceral clefts. 



In Amphioxus the respiratory region extends close up to the 

 opening of the hepatic diverticulum, and therefore to a position 

 corresponding with the commencement of the intestine in higher 

 types. In the craniate Vertebrata the number of visceral clefts 

 has become reduced, but from the extension of the visceral clefts 

 in Amphioxus, combined with the fact that in the higher Verte- 

 brata the vagus nerve, which is essentially the nerve of the 

 branchial pouches, supplies in addition the walls of the oesophagus 

 and stomach, it may reasonably be concluded, as has been pointed 

 out by Gegenbaur, that the true respiratory region primitively 

 included the region which in the higher types forms the 

 oesophagus and stomach. 



In Ascidians the respiratory sack is homologous with the 

 respiratory tract of Amphioxus. 



The details of the development of the branchial clefts in the 

 different groups of Vertebrata have already been described in 

 the systematic part of this work. 



In all the Ichthyopsida the walls of a certain number of 

 clefts become folded ; and in the mesoblast within these folds a 

 rich capillary network, receiving its blood from the branchial 

 arteries, becomes established. These folds constitute the true 

 internal gills. 



