THE BODY CAVITIES, DIAPHRAGM AND MESENTERIES 



183 



mesocardium, attaching the heart to the ventral body wall, disappears 

 and the right and left limbs of the U-shaped cavity become confluent, 

 ventral to the heart. The result is a single, large pericardial chamber, 

 the long axis of which now lies in a dorso-ventral plane nearly at right 

 angles to the plane of the pleuro-peritoneal cavities, and connected with 

 them dorsally by the right and left pleuro-peritoneal canals. 



The division of the primitive ccelom into separate cavities is accom- 

 plished by the development of three membranes that join in a Y-shaped 

 fashion (Figs. 194 and 195) : (i) the septum transversum, which separates 



Pericardial cavity 



Somatopleiire 



Septum transversum 



Liver trabeculx 

 Hepatic diverticulum 



Yolk stalk 



Bulbus cordis 



Dorsal mesocardium 



Sinus venosus 



Lateral mesocardium 

 Common cardinal vein 



Umbilical vein 



Vitelline vein overlying 

 the stomach 



Pleuro-peritoneal canal 



Peritoneal cavity 



FIG. 191. Reconstruction cut at the left of the median sagittal plane of a 3 mm. human em- 

 bryo, showing the body cavities and septum transversum (Kollmann). 



incompletely the pericardial 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 con- 

 tains the abdominal viscera. 



The Septum Transversum. The vitelline veins, on their way to the 

 heart, course in the splanchnic mesoderm lateral to the fore-gut. In 

 embryos of 2 to 3 mm. these large vessels bulge into the ccelom until they 

 meet and fuse with the somatic mesoderm (Figs. 88 and no). Thus there 

 is formed caudal to the heart a transverse partition filling the space be- 

 tween the sinus venosus of the heart, the gut, and the ventral body wall, 

 and separating the pericardial and peritoneal cavities from each other 



