PERICARDIUM, PLEUROPERITONEUM, DIAPHRAGM AND MESENTERIES. 377 



The Pericardium and Pleura. Since the pericardial cavity represents a 

 portion of the original ccelom, the lining of the cavity must be a derivative of 

 either the parietal or the visceral layer of mesoderm or of both. The common 

 mesentery in which the heart develops is derived from the visceral layer. Con- 

 sequently the epicardium is a derivative of the visceral mesoderm (Fig. 203). 

 The pericardium is derived from three regions of mesoderm. The greater 

 part is derived from the parietal mesoderm, since the body wall which is com- 

 posed of parietal mesoderm is also primarily the wall of the pericardial cavity. 

 A small dorsal portion is probably derived from the mesoderm at the root of the 

 dorsal mesocardium (Fig. 203). The septum transversum primarily forms 

 the caudal wah 1 of the pericardial cavity, and, since the septum is a derivative 

 of the visceral layer, the caudal wall is derived from this layer. The three 

 portions are, of course, continuous. 



The lungs first appear in the common mesentery as an evagination from the 

 primitive gut (Fig. 320, p. 360). As they develop further they grow into the 

 pleural cavities, pushing a part of the mesentery before them. This part of 

 the mesentery thus invests the lungs and forms the visceral layer of the pleura 

 which is therefore a derivative of the visceral mesoderm. The parietal layer of 

 the pleura is a derivative of the parietal mesoderm, since the wall of the pleural 

 cavity is primarily the body wall. 



The lining of all these cavities is at first composed of mesothelium and 

 mesenchyme. The latter is transformed into the delicate connective tissue of 

 the serous membranes, and the mesothelium becomes the mesothelium of 

 the membranes. 



The Omentum and Mesentery. 



From the septum transversum (or diaphragm) to the anus the gut is sus- 

 pended in the ccelom (or abdominal cavity) by means of the dorsal mesentery. 

 This is attached to the dorsal body wall along the medial line and lies in the 

 medial sagittal plane (Fig. 301; compare with Fig. 235). On the ventral side of 

 the gut a mesentery is lacking from the anus to a point just cranial to the yolk 

 stalk (p. 371). There is, however, a small ventral mesentery extending a short 

 distance caudally from the septum transversum. On account of its relation to 

 the stomach this is known as the ventral mesogastrium (Fig. 301). These two 

 sheets of tissue, the dorsal and ventral mesenteries, are destined to give rise to 

 the omenta and mesenteries of the adult. Owing to the enormous elongation of 

 the gut and its extensive coiling in the abdominal cavity, the primary mesen- 

 teries are twisted and thrown into many folds which enclose certain pockets or 

 bursas. Furthermore, certain parts of the gut which are originally free and 

 movable become attached to other parts and to the body walls through fusions 

 of certain parts of the mesentery with one another and with the body walls. 



