CHAPTER XI 



THE BODY-CAVITIES, MESENTERIES AND SEPTUM 

 TRANSVERSUM 



THE 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 pleuro peritoneal 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 

 coelome 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 coelome (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 



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