THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 567 



The pericardio-thoracic cavity begins very early to be sharply 

 marked off from the future abdominal cavity by a transverse fold 

 (figs. 313, 314 z + l), which begins at the front [ventral] and lateral 

 walls of the trunk, and the free edge of which projects dorsal wards 

 and median wards (fig. 314 z + l) into the primitive body-cavity. It 

 marks the course which the terminal part of the vena omphalo- 

 mesenterica takes in order to reach the heart. Subsequently there 

 are found imbedded in the fold all of the venous trunks which empty 

 into the atrial sinus of the heart (figs. 313, 314), the omphalo- 

 mesenteric and umbilical veins and the Cuvierian ducts (dc), which 

 collect the blood from the walls of the trunk. Therefore the formation 

 of the transverse fold is most intimately connected with the development 

 of the veins. It takes the name of septum transversum (massa 

 transversa, USKOW), and has the form of a transverse bridge of 

 substance uniting the two lateral walls of the trunk (fig. 313), which 

 inserts itself between the sinus venosus and the stomach, and 

 is united with both as well as with the ventral mesentery. Its 

 posterior portion (fig. 314 z + l) contains abundant embryonic con- 

 nective tissue and blood-vessels, and constitutes a mass described as 

 prehepaticus (Yorleber), since the two liver-sacs (fig. 313 Lb + Lbg) 

 grow out from the duodenum into it and produce the hepatic 

 cylinders. In proportion as this takes place, and the hepatic 

 cylinders spread out from the ventral mesentery laterally into the 

 septum transversum, the latter increases in thickness and now 

 embraces two different fundaments, in front, a plate of substance 

 in which the Cuvierian ducts and other veins run to the heart (the 

 primary diaphragm) ; behind, the two lobes of the liver, which produce 

 ridges that project into the body cavity. 



By means of the septum transversum the pericardio-thoracic and 

 the abdominal cavities are almost completely separated (fig. 314). 

 There remain only two narrow canals (brh) (thoracic prolongations 

 of the abdominal cavity, His), which establish a connection behind 

 with the abdominal cavity at either side of the intestinal tube and 

 its dorsal mesentery. The two canals (brh) receive the two funda- 

 ments of the lungs (Ig) when they grow out from the ventral wall 

 of the intestinal tube. They afterwards become the two thoracic or 

 pleural cavities (brh), whereas the larger cavity communicating with 

 them (M), in which the heart has developed, becomes the pericardial 

 chamber. The latter takes up the whole ventral side of the embryo; 

 the thoracic cavities, on the contrary, lie quite dorsal next to the 

 posterior wall of the trunk. 



