868 THE DEVELOPMENT OF THE COELOMIC CAVITIES 



formed by the dorsal closing folds, are inclined to a great degree, with the 

 ventral end of the primary septum transversum considerably more posterior 

 in position than the dorsal edge of the dorsal membranes. Consequently, a 

 secondary recess or pocket is formed on either side anterior and dorsal to 

 the septum transversum. This secondary recess occurs on either side of the 

 gut, and, into each of these recesses, a lung extends in many reptiles and in 

 those amphibia which possess lungs. In this pocket also lie certain of the air 

 sacs of birds. Thus, the general cavity back of the pericardioperitoneal mem- 

 brane or secondary septum transversum (i.e., the primary septum transversum 

 plus the two dorsal membranes, formed by the dorsal closing folds) is known 

 as the pleuroperitoneal cavity in amphibia and many reptiles. In birds (see 

 below), the respiratory part of the lung becomes enclosed dorsally near the 

 vertebrae within a separate pleural cavity, separated from the peritoneal cavity 

 by the dorsal diaphragm (fig. 362J). The thin air sacs of the bird's lung 

 (Chap. 14) project from the lung through the dorsal diaphragm into the 

 peritoneal cavity and also into certain of the bones. In the turtle group, among 

 the reptiles, a dorsal diaphragm is developed below each lung, segregating 

 the lungs partly within dorsal cavities, thus simulating the bird condition. 



D. Formation of the Coelomic Cavities in Mammals 



In the mammalia, a pronounced caudal migration of the heart, liver, and 

 developing diaphragm occurs. Also, as in birds, a further morphogenetic fea- 

 ture is present which results in the development of a pleural cavity for each 

 lung in addition to the peritoneal and pericardial cavities present in fishes, 

 amphibians, and reptiles. Thus it is that the development of two partitioning 

 membranes on either side of the gut tube, the pleuropericardial membranes, 

 which correspond to the dorsal closing membranes mentioned above, together 

 with two additional membranes, the pleuroperitoneal membranes, are neces- 

 sary to effect the division of the primitive splanchnocoelic coelom into the 

 four main coelomic cavities in the Mammalia. 



1. Formation of the Pleuropericardial Membrane 



It so happens that the anterior cardinal vein develops slightly in advance 

 of the posterior cardinal vein. As a result the common cardinal vein, which 

 develops from the caudal end of the primitive anterior cardinal vein, travels 

 along the lateral body wall in an inclined plane to reach the area of the lateral 

 mesocardium and sinus venosus of the heart. This inclined pathway of the 

 common cardinal vein is characteristic of the vertebrate embryo. As the 

 common cardinal vein increases in size, a lateral ridge or elongated bulge 

 is formed along the lateral body wall. This ridge projects inward into the 

 coelomic cavity and inclines caudo-ventrally to reach the dorsal edge of the 

 area of the primitive septum transversum (fig. 363D). 



In the mammals, the mesonephric folds (ridges), in which the mesonephric 



