EMBRYOLOGY 



19 



splanchnocoele extends forward, dorsal to the pericardium and into 

 this recess the lungs extend. Then a pair of folds grow ventrally 

 from the dorsal wall of the body cavity until they meet the septum 

 transversum, thus cutting off a pair of pleural cavities containing the 

 lungs from the rest of the splanchnocoele (fig. ii,C). As the lungs 

 increase in size, the pleural cavities extend, at first lateral to the 

 pericardium, and later, ventral to it, until at last the pleural cavities 

 of the two sides are separated ventrally by their two walls which form 

 a mesenterial-like membrane, the mediastinum. The diaphragm 

 undergoes several shiftings of position before reaching its final place. 

 The dorsal parts of the diaphragmatic muscles are derived from the 

 original folds, the ventral come from the rectus abdominis muscle of 

 the ventral abdominal wall. 



The tunica serosa lining the various divisions of the coelom has special names 

 in each. The pericardial and pleural cavities are lined with pericardium and 



Fig. II. — Diagram showing the relations of the coelomic cavities (black) in A , fishes, 

 B, amphibians and sauropsida; and C, in mammals; L, liver; P, lungs in C in pleural 

 coelom; S, septum transversum; Z), diaphragm. 



pletira respectively, that part of the pericardial serosa covering the heart itself 

 being frequently called the epicardium. The body or peritoneal cavity is lined 

 with peritoneum which extends over the mesenteries, the digestive tract and the 

 excretory and reproductive organs. 



The body cavity is not always completely cut off from the external 

 world. In the ichthyopsida the urinary ducts which lead to the 

 exterior frequently open into the body cavity by the nephrostomes 

 (fig. 12), and, in the females of all vertebrates with true oviducts, 

 including the mammals, these tubes connect the peritoneal cavity 

 with the outer world. Many fishes have short tubes (pori abdom- 

 inales) leading from the body cavity to the exterior, opening near the 

 vent. These abdominal pores occur in cyclostomes, many elasmo- 



