FORMATION OF COELOMIC CAVITIES IN MAMMALS 869 



kidneys develop, are large and project downward into the coelomic cavity. 

 The anterior ends of the mesonephric ridges continue along the lateral body 

 wall on either side and follow an inclined plane antero-ventrally to the dorsal 

 edge of the primitive septum transversum (fig. 363D). Two lateral body folds 

 or ridges, which incline toward and fuse with the dorsal edge of the primitive 

 septum transversum, are produced in this manner on either side. These folds 

 are an anterior lateral body fold or ridge, overlying the common cardinal 

 vein, and a posterior lateral body fold, which represents the antero-ventral 

 continuation of the mesonephric ridge as it inclines ventrally to join the lateral 

 edge of the primitive septum transversum (fig. 363D). A V-shaped pocket is 

 formed between these two ridges. This pocket represents the primitive pleural 

 cavity or pocket. The apex of this V-shaped pocket unites with the primitive 

 septum transversum. As the lung buds grow out posteriorly below the foregut, 

 each projects into a pleural pocket (fig. 363F). 



The formation of the pleuropericardial membrane is effected by an ingrowth 

 of tissue along the edge of the anterior, lateral body fold, the fold that overlies 

 the common cardinal vein. This ingrowing tissue forms a secondary ridge, 

 known as the pulmonary ridge, which continues to grow mesad below the 

 developing lung until it reaches the splanchnopleure of the esophagus with 

 which it fuses. A pleuropericardial membrane, in this way, is established which 

 separates the pericardial cavity below from the pleural cavity above (fig. 

 363E-G). The pleuropericardial membranes probably are homologous with 

 the dorsal closing folds of the secondary septum transversum of the vertebrates 

 below the mammals. 



2. Development of the Pleuroperitoneal Membrane 



As mentioned previously, the cephalic end of the mesonephric ridge projects 

 forward and ventrad along the lateral body wall to unite with the primitive 

 septum transversum to form the posterior, lateral body fold. The medial 

 growth of this posterior, lateral body fold and ultimate fusion with a small 

 splanchnopleural outgrowth, the splanchnopleural fold, forms a second par- 

 titioning membrane, the pleuroperitoneal membrane, which separates the 

 pleural cavity from the general peritoneal cavity (fig. 363E-H). Contributions 

 of the somatic mesoderm to the lateral body-fold tissue are significant in the 

 formation of the pleuroperitoneal membrane. It is to be noted that the primi- 

 tive pleural cavities of the mammalian embryo are small and dorsally placed, 

 one on either side of the gut and dorsal to the pericardial cavity. Their later 

 expansion is described below. To summarize the partitioning process of the 

 primitive coelom in mammals, we find that the following membranes are 

 formed: 



( 1 ) the primary septum transversum, 



(2) the two dorsal closing folds or pleuropericardial membranes, and 



(3) two pleuroperitoneal membranes. 



