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 second component 
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- 
wall 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 
