EMBRYOLOGY. 15 



the ectoderm by the mesenchyme. In this way the ccelom comes to 

 have two thick walls. That on the outer side, consisting of ectoderm, 

 muscles and peritoneal lining, is called the somatopleure (so), that of 

 peritoneum and digestive wall is the splanchnopleure (sp). 



For convenience the different mesenterial structures have separate names. 

 As the digestive tract r/ecomes coiled, the different parts of it are connected by 

 similar membranes which are called omenta (om). The dorsal mesentery is sub- 

 divided into regions supporting the different portions of the digestive tract. 



FIG. 9. Diagrammatic transverse section of a vertebrate to illustrate mesenteries, 

 omen turn and downward growth of the myotomes. al, alimentary tract; ao, aorta; c t 

 coslom; ec, ectoderm; dmes, dorsal mesentery; my, myotome; nc, notochord; neph, nephro- 

 tome; o, omen turn; sc, spinal cord; so, sp, somatic and splanchnic layers of mesothelium; 

 vmes, ventral mesentery. 



Thus there is a mesogaster for the stomach, a mesentery proper for most of the 

 intestine, and mesocolon and mesorectum for colon and rectum respectively. 

 On the ventral side there is a mesohepar, bounding the liver to the ventral body 

 wall. In the same way the omenta are distributed into hepato-duodenal, gas- 

 tro-hepatic (small omentum), etc., while in mammals there is a great omentum, 

 a double fold of mesogaster and mesocolon which connects the stomach with the 

 transverse colon. 



Similar folds are formed in connection with other organs. Thus the heart 



