EMBRYOLOGY 



17 



tract to the dorsal region of the body (fig. 9, dmes). With farther 

 development mesenchyme grows in between the two serosal walls of 

 the dorsal mesentery, uniting them and forming a tissue through 

 which blood-vessels, lymphatics and nerves reach the digestive tract. 

 In many vertebrates the dorsal mesentery persists throughout life, 

 but in some it becomes reduced, persisting chiefly in those places 

 where the vessels extend to the alimentary canal. The ventral 

 mesentery, on the other hand, almost entirely disappears (fig. 10), 

 the parts persisting being recognized as mesenterial in origin only by 

 following their development. With the breaking down of the ventral 

 mesentery the metacceles of the two sides are in open communication 

 with each other and form the well-known ' body cavity' of the adult. 



Fig. 10. — Diagrammatic section of a vertebrate to show the relation of the body 

 walls, etc., av, aorta; c, coelom; c, ectoderm; c^, epaxial muscles; g, gonads; ha, haemal 

 arch; hp, hypaxial muscles; i, intestine; mes, mesentery; n, nephridium; 0, omentum; r, 

 rib; p, somatopleure; sp, splanchnopleure; v, vertebra. 



For convenience of description different parts of the mesenteries have re- 

 ceived special names according to the organs supported. The persisting remnant 

 of the ventral mesentery occurs in the region of the liver where it is called the 

 mesohepar, and in the ichthyopsida it may carry blood-vessels from the body 

 wall to that organ. In the dorsal mesentery the regions are the mesogaster, 

 mesentery proper, mesocolon, mesorectum, etc., accordingly as they support 

 the stomach, intestine, colon, rectum and the like. 



In all vertebrates except the cyclostomes the alimentary canal is bent on 

 itself and the folds are connected with each other by portions of the serosa, and 

 these mesenterial-like structures are called omenta (fig. 10, 0). There may be 

 several of these omenta, their names usually indicating the parts of the digestive 

 tract which they connect. Thus the gastrohepatic omentum connects stomach 



