MAMMALIAN DIAPHRAGM 871 



E. Development of Independent Pericardial Walls 



1. The Arrangement of the Parietal Pericardial Wall in Fishes 



The parietal pericardium of the fish embryo is fused with the lateral body 

 wall. The caudal area of the sinus venosus is associated intimately with the 

 anterior wall of the septum transversum. This condition is a primary one in 

 all vertebrate embryos. It is retained in the adult fish. 



2. Formation of an Independent Parietal Pericardial Wall 



in the Chick 



In the chick, two main processes occur in development which separate the 

 septum transversum from the liver, and also the parietal pericardial mem- 

 brane from the lateral body walls. These processes are: 



(a) The peritoneal cavity on either side of the liver grows forward and 

 separates the cardiac or anterior face of the liver from the posterior 

 face of the septum transversum, with the exception of the area where 

 the veins from the hepatic region perforate the septum. This process 

 frees the septum transversum from the liver surface and permits it to 

 function as a part of the pericardial sac as indicated in figure 294G-4; 

 G-5. 



(b) The extending peritoneal coelom not only separates the liver from the 

 posterior face of the septum transversum, but it continues anteriad 

 followed by the liver lobes along the ventral and lateral aspects of 

 the body wall and splits the membranous pericardium away from the 

 lateral body wall. Ventrally, a median septum unites the pericardium 

 with the body wall (fig. 362J). 



3. Formation of the Independent Parietal Pericardial Wall 

 IN Amphibians and Reptiles 



A somewhat similar process to that described for the chick obtains in rep- 

 tiles and, to a modified extent, in amphibia. 



Fig. 364 — Continued 



cavities of a 10 mm. pig embryo showing position of the primitive mediastinum. (B-2) 

 Later mediastinal area development portraying adult position (black area) of the medi- 

 astinum. (Based on the cat.) Observe that fig. 364 (A) is an intermediate condition 

 between figs. 364 (B-1) and 364 (B-2). (C) Probable origin of parts of the mam- 

 malian diaphragm. (D) The caudal migration of the septum transversum and devel- 

 oping diaphragm during development. 2-position = embryo of 2 mm.; 24-position = 

 24 mm. embryo. (Redrawn from F. P. Mall. 1910, Chap. 13, Vol. 1, Manual of Human 

 Embryology, Lippincott. Philadelphia.) (E-H) Development of the mesenteries and 

 omental bursa or lesser peritoneal cavity in the human. The cross-lined areas in H 

 show areas of the mesentery which fuses with the body wall. The arrows in F-H denote 

 development of the lesser peritoneal cavity. 



