THE BODY CAVITIES. DIAPHRAGM AND MESENTERIES 



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 transfer sum, which separates 



Pericardial cavity 



Somj.topl:ure 



Septum transversu 



Liver trabeculx 

 Hepatic diverticulnm 



Yolk stalk 



Bulhus cordis 



Dorsal ntesocardium 



Sinus venosus 



lateral mesocardium 

 Common cardinal vein 



Umbilical vein 



Vitelline vein overlying 

 the stomach 



Pleuro-peritoneal canal 



Peritoneal cavity 



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

 bryo, showing the body cavities and septum trans versum (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 



