l82 



THE ENTODERMAL CANAL AND THE BODY CAVITIES 



the cievelopment of three membranes which join in a -< -shaped fashion (Figs. 

 194 and 195) : (1) the septum transversum, which separates incompletely the peri- 

 cardial and pleural cavities from the peritoneal cavities; (2) the paired pleuro- 

 pericardial membranes, which complete the division between pericardial and 

 pleural cavities; (3) the paired pleuro- peritoneal membranes, which complete the 

 partition between each pleural cavity, containing the lung, and the peritoneal 

 cavity, which contains the abdominal viscera. 



The Septum Transversum. — The vitelhne veins, on their way to the heart, 

 course in the splanchnic mesoderm lateral to the fore-gut. In embryos of 2 to 3 



Pericardial cavity 



Somatopleiire 



Septum transversum 



Liver Irabeculx 

 Hepatic diverticulum 



Yolk stalh 



BiUbus cordis 



Dorsal mesocardium 



Sinus venosus 



Lateral mesocardium 

 ^ — Common cardinal vein 



Umbilical vein 



Vitelline vein overlying 

 stomach 



Pleuro- peritoneal canal 



Peritoneal cavity 



Fig. 191.— Reconstruction cut at the left of the median sagittal plane of a 3 mm. human embryo, showing 

 the body cavities and septum transversum (KoUmajm). 



mm. these large vessels bulge into the ccelom until they meet and fuse with the 

 somatic mesoderm (Figs. 88 and 110). Thus there is formed caudal to the heart 

 a transverse partition filhng the space between the sinus venosus of the heart, 

 the gut, and the ventral body wall, and separating the pericardial and peri- 

 toneal cavities from each other ventral to the gut. This mesodermal partition 

 was termed by His the septum transversum. In Fig. 191 it comprises both a 

 cranial portion (designated "septum transversum"), which is the anlage of a large 

 part of the diaphragm, and a caudal portion, the ventral mesentery, into which 

 the liver grows. 



At first the septum transversum does not extend dorsal to the gut, but leaves 



