866 THE DEVELOPMENT OF THE COELOMIC CAVITIES 



formation of the primitive liver-septum transversum complex is similar to that 

 described above. 



2) Formation of the Liver-Septum Complex in the Human Embryo. In the 



developing human embryo, medial growths on either side from the somato- 

 pleural mesoderm occur in the region caudoventral to the forming sinus ven- 

 osus, and below the developing gut tube. In this way ,a primitive transverse sep- 

 tum is formed below the lateral mesocardia and between the entoderm of the gut 

 and the caudal wall of the parietal pericardium (fig. 362F, G). This septum 

 fuses with the lateral mesocardia and caudal wall of the parietal pericardium. 

 However, when the evaginating liver rudiment grows ventrad and forward into 

 the splanchnopleural tissue below the gut, it ultimately appropriates the previ- 

 ously formed transverse septum as its anterior aspect. Consequently, the general 

 result of the two methods is the same, namely, the transverse septum in its 

 earlier stages of development appears as the thickened anterior face of the 

 liver associated with the lateral mesocardia (figs. 261 A; 362H, I). 



c. Formation of the Primary Septum Transversum 



After the liver-septum transversum complex has been established and the 

 potential ventral parietal recesses are closed by either of the two methods 

 described above, the next stage in the development of the primitive septum 

 transversum is correlated with the forward expansion of the peritoneal coelom 

 around the sides and anterior face of the liver. In doing so, the peritoneal 

 coelom on either side of the liver extends anteriad and mesiad and thus be- 

 comes involved in a secondary separation of the liver from the lateral and 

 ventral body wall and also from the anterior face of the liver itself which be- 

 comes the primary septum transversum (fig. 363 A, B). A separation does not 

 occur in the area traversed by the veins passing from the liver to the sinus 

 venosus or slightly dorsal to this area. Here the liver remains attached directly 

 to the septum transversum and is suspended literally from it. This attaching 

 tissue forms the coronary ligament of the liver. The ingrowth of the two 

 coelomic areas on either side of and ventral to the liver, by apposition of 

 the coelomic epithelium in the median plane, forms a secondary ventral mesen- 

 tery of the liver. This secondary ventral mesentery or falciform ligament ties the 

 liver to the mid-ventral area of the body wall and to the septum transversum. 

 (Note: The terms primary septum transversum and peritoneopericardial mem- 

 brane are synonymous.) 



C. Coelomic Changes in Fishes, Amphibians, Reptiles, and Birds 



1. In Fishes 



In the adult shark, and fishes in general, the fully developed adult form 

 of the septum transversum forms a complete partition between the pericardial 

 cavity and the peritoneal cavity. In fishes the pericardial cavity in the adult 

 fish, as in the embryo, extends laterally and ventrally to the body wall in a 



