CCELOM. 



121 



walls into epimere, mesomere and hypomere, the ccelom undergoes a 

 corresponding division. That portion in the epimere is divided into 

 a series of cavities in the myotomes (myocoeles), which are eventually 

 obliterated (p. 126); the portions in the mesomere persist only as the 

 lumina of the excretory organs and their ducts, described under the 

 urogenital system; while that part of the original ccelom in the hypomere 

 gives rise to all of the permanent body cavities of the adult. 



The hypomeres gradually descend between the ectoderm and the 

 entoderm (fig. 130) until their lower margins meet, ventral to the diges- 

 tive tract. In this way the latter 

 becomes surrounded by a pair of 

 cavities, the splanchnocceles or body 

 cavities of the adult. Each is 

 bounded by epithelium, the tunica 

 serosa, in .which an outer or somatic 

 wall is turned toward the ectoderm, 

 while the inner or splanchnic wall 

 adjoins the alimentary canal. Later, 

 when the muscle plates extend down- 

 ward (fig. 135), they unite ectoderm 

 and serosa into the outer body wall, 

 the somatopleure, while the invasion 

 of mesenchyme unites the splanchnic 

 serosa with the entoderm into a similar 

 splanchnopleure . 



Mesenteries. As has just been 

 stated the walls of the two ccelomic 



FIG. 130. Diagram of early meso- 

 derm, showing the zones, e, h, mm, epi-, 

 hypo-, and mesomeres, the walls of the 

 coelom, dm, vm, to form the dorsal 

 and ventral mesenteries. .4, alimen- 

 tary canal; e, ectoderm; so, sp, soma- 

 topleure and splanchnopleure. 



cavities meet below the digestive tract, thus forming a double membrane 

 running lengthwise of the body and binding the alimentary canal to the 

 ventral body wall. This membrane is called the ventral mesentery. 

 In a similar way the splanchnic walls meet above the digestive tract 

 forming a dorsal mesentery. These mesenteries are eventually more 

 than double serosal walls, since mesenchyme comes in between, uniting 

 them and affording a tissue through which blood-vessels, lymphatic 

 vessels and nerves can reach the digestive organs. 



For convenience of reference different parts of these mesenteries have received 

 special names, according to the organs supported. The ventral mesentery 

 usually almost entirely disappears, only a small portion persisting in the region 

 of the liver, the mesohepar, which, in the ichthyopsida may carry blood-vessels 



