CHAPTER XI 
THE BODY-CAVITIES, MESENTERIES AND SEPTUM 
TRANSVERSUM 
Tue development of these parts is one of the most difficult 
subjects in embryology, involving, as it does, complex relations 
between the viscera, vascular system, and primitive body-cavity, 
on which the definitive relations of the body-cavities and mesen- 
teries depend. 
The pericardial and pleuroperitoneal cavities are completely 
separated in all vertebrates excepting Amphioxus, cyclostomes 
and some Selachii and ganoids, in which narrow apertures exist 
between the two. The pleural and peritoneal divisions of the 
ccelome of the trunk communicate widely in amphibia; among 
reptiles completely closed pleural cavities are found apparently 
only in Crocodilia; in birds and mammals they are completely 
closed. 
As we have seen, in the early embryo of the chick there is 
free communication between all parts of the body-cavity. We 
have to consider, therefore, (1) the separation of the pericardial 
and pleuroperitoneal cavities, (2) the separation of pleural and 
peritoneal cavities, and (3) development of the mesenteries. 
I. THe SEPARATION OF THE PERICARDIAL AND PLEUROPERI- 
TONEAL CAVITIES 
The pericardial cavity proceeds from the cephalic division of 
the primitive ecelome (parietal cavity of His). We may review 
its primitive relations as follows (stage of 10 somites; see Chap. 
V): it contains the heart which divides it into right and left 
parts so long as the dorsal and ventral mesocardia persist; these, 
however, disappear very early. Laterally, the parietal cavity 
communicates with the extra-embryonic body-cavity (Figs. 53 
and 54); posteriorly it is bounded by the wall of the anterior 
intestinal portal (Fig. 67), on which the heart is seated like a 
333 
