COELOMIC CHANGES IN FISHES. AMPHIBIANS, REPTILES, AND BIRDS 867 



fashion similar to that of the peritoneal cavity. Also, the heart continues to 

 lie posterioventrally to the pharyngeal region in a manner very similar to that 

 of the basic, embryonic body plan (fig. 294G-I). 



In the formation of the adult, piscine, septum transversum from the primary 

 transverse septum two membranous partitions are developed which close the 

 dorsal parietal recesses or the openings above the lateral mesocardia. These 

 partitions are called the dorsal closing folds and they arise as follows: 



The splanchnopleural tissue on either side of the foregut, just anterior to 

 the stomach rudiment and above the primitive septum transversum, forms a 

 thin fold of tissue. This fold grows laterad and ventrad and fuses ultimately 

 with the lateral mesocardium and the somatopleuric tissue, which overlies the 

 common cardinal vein, as this vein travels caudo-ventrally along the body wall 

 to reach the lateral mesocardium and the sinus venosus. As a result of this 

 splanchnopleuric and somatopleuric fusion of tissues with the dorsal edge of 

 the primary septum transversum a dorsal closing fold is formed on either side 

 of the esophagus, and the two dorsal parietal recesses are obliterated, separat- 

 ing completely the pericardial cavity from the peritoneal cavity (fig. 362D). 

 However, a small pericardioperitoneal opening may be left below the esophagus 

 in the shark. 



The secondary septum transversum thus formed is a thickened transverse 

 partition, composed of two walls, an anterior pericardial wall and a posterior 

 peritoneal wall, with a loose tissue layer between these two coelomic mem- 

 branes. The liver is suspended from the peritoneal or caudal aspect of the 

 septum transversum in the region of the coronary ligament, while the posterior 

 end of the sinus venosus is apposed against the anterior or pericardial face of 

 the transverse septum. The common cardinal and other converging veins of 

 the heart utilize the substance of the septum transversum as a support on their 

 way to the sinus venosus. The hepatic veins (the right and left, embryonic 

 vitelline veins) pass through the coronary ligament on their journey to the 

 sinus venosus. 



2. In Amphibians, Reptiles, and Birds 



The conversion of the primary septum transversum in amphibians, reptiles, 

 and birds into the secondary or adult septum transversum occurs essentially 

 as described above. A dorsal closing fold, obliterating the dorsal parietal 

 recess on either side of the gut, is developed, although, in reptiles and birds, 

 the inward growth and contribution of somatopleuric tissue overlying the 

 common cardinal ridge is more important than in fishes in effecting this closure. 



However, one must keep in mind an important fact, namely, that, in am- 

 phibia, reptiles and birds, there is an extensive caudal migration of the heart, 

 septum transversum, and liver complex from their original cephalic position 

 just posterior to the pharyngeal area. This caudal migration produces a con- 

 dition in which the primary septum transversum and the dorsal membranes, 



