THE BODY-CAVITIES 337 



cates with the peritoneal cavity around the median mass of the 

 septum transversum beneath the lateral mesocardia. 



2. The lateral mesocardia constitute the seconil <^omponent 

 of the septum transversum. At the stage of sixty hours they 

 are nearly round in section. At eighty-six hours the substance 

 posterior to the duct of Cuvier begins to thicken (Fig. 192) so 

 that the section is no longer round but elongated towards the 

 umbilicus. They still extend almost transversely to the lateral 

 body-wall. However, the retreat of the heart backwards soon 

 changes their direction (Fig. 193 A) so as to form a long oblique 

 partition between the pericardium and the dorsal parietal recess^ 

 the direction of the ducts of Cuvier being changed at the same 

 time. The lateral mesocardia are directly continuous with the 

 anterior portion of the median mass of the septum transversum. 



3. The lateral closing folds arise as ridges of the lateral body- 

 w^all extending obliquely from the primary ventral ligament of 

 the liver upwards and forwards to the lateral mesocardia. They 

 arise along the course of the umbilical veins which open at first 

 into the ducts of Cuvier. As the lateral closing folds develop 

 first at their anterior ends, they appear as direct backward 

 prolongations of the lateral mesocardia. They fuse with the 

 lateral ventral surface of the liver (median mass of the septum 

 transversum), and when they are completed back to the primary 

 ventral ligament of the liver, they completely close the ventral 

 communication of the pericardium with the peritoneal cavity. 

 They mark out a triangular area on the cephalic face of the liver 

 with postero-ventral apex and antero-dorsal base, which forms 

 the median portion of the posterior wall of the pericardium (cf. 

 Fig. 193 A). At six days the ventral communication of the 

 pericardium is reduced to a very small opening, and at eight days 

 it is entirely closed. 



Closure of the Dorsal Opening of the Pericardium. As already 

 noted the pericardial cavity communicates with the peritoneal 

 cavity above the lateral mesocardia by way of the dorsal 

 parietal recesses, which are destined to form a large part of the 

 pleural cavities. We have, therefore, to consider next the closure 

 of the aperture between the pleural and pericardial cavities. 

 We have already seen that the heart shifts backwards very rapidly 

 between the third and sixth days, and this draWs out the lateral 

 mesocardia in an oblique plane directed from dorsal anterior to 



