CHAPTER XIV. 



THE DEVELOPMENT OF THE CGELOM (PERICARDIAL 



PLEURAL AND PERITONEAL CAVITIES), THE 



PERICARDIUM, PLEUROPERITONEUM, 



DIAPHRAGM, AND MESENTERIES. 



In the Chapter on the development of the germ layers, it is stated that the 

 peripheral part of the mesoderm splits into two layers, an outer or parietal, and 

 an inner or visceral (Fig. 81; see also p. 83). The parietal layer of mesoderm 

 and the ectoderm constitute the somatopleure. The visceral layer and the 

 entcderm constitute the splanchnopleure (Fig. 81). The cleft or cavity 

 that appears between the parietal and visceral layers is the ccelom or body 

 cavity and is lined with a layer of flattened mesodermal cells known as the 

 mesothelium. It will be remembered that in the earlier stages of development a 

 portion of the embryonic disk becomes constricted off from the yolk sac to form 

 the simple cylindrical body (p. 137). Along each side of the axial portion of the 

 germ disk, and also at its cephalic and caudal ends, the germ layers bend ven- 

 trally and then medially until they meet and fuse in the midventral line (p. 141). 

 In this way a part of the somatopleure forms the lateral and ventral portions of 

 the body wall (Fig. 141). At the same time the axial portion of the entoderm is 

 bent into a tube which is closed at both ends the primitive gut and is then 

 pinched off from the rest of the entoderm except at one point, where the cavity 

 of the gut remains in communication with the cavity of the yolk sac. The 

 splanchnic mesoderm adjacent to the entoderm on each side comes in contact 

 and fuses with the corresponding portion from the opposite side, thus forming 

 a sheet of tissue which encloses the primitive gut and also forms a partition be- 

 tween the two parts of the coelom. This sheet of tissue is the common mesentery 

 and is attached to the dorsal and ventral body walls along the medial line. 

 The portion between the gut and the dorsal body wall is the dorsal mesentery, 

 the portion between the gut and the ventral body wall is the ventral mesentery. 

 Thus the gut is suspended in the common mesentery (Figs. 235 and 320). 



When portions of the somatopleure and splanchnopleure are bent ventrally 

 the coelom between the portions is naturally carried with them. This part of 

 the coelom thus becomes enclosed within the cylindrical body and constitutes 

 the intraembryonic or simply the embryonic coelom (body cavity proper). The 

 part of the coelom which, while the germ layers were still flat, was situated more 

 peripherally constitutes the extraembryonic coelom or eococcelom (extraembryonic 



370 



