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LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



opening to the exterior, the blastopore, which is situated at the future posterior end of the 

 embryo. This entodermal tube persists as the lining of the adult digestive tract and of all of 

 its derivatives; to it there are added other layers (connective tissue and muscular layers) 

 derived from the splanchnic mesoderm of the hypomere, which, it will be recalled, folds around 

 the archenteron. The adult digestive tract thus consists of a thick-walled tube, composed of 



a 



FIG. 45. Diagrams to illustrate the divisions of the coelom in the various vertebrate classes. 

 The transverse septum and its derivatives are indicated by thick lines. A , fishes, showing the division 

 of the coelom into pericardial cavity a and pleuroperitoneal cavity g by means of the transverse septum d. 

 B, urodeles, similar to fishes with the addition of the lung h which projects into the pleuroperitoneal 

 cavity g. C, turtle; the pericardial cavity a has descended posteriorly until it lies ventral to the anterior 

 part of the pleuroperitoneal cavity g; the anterior face of the transverse septum d has now become part 

 of the wall of the pericardial sac; the lung h is retroperitoneal. D, early stage of mammals, showing the 

 beginning of the coelomic fold (pleuroperitoneal membrane) j descending from the dorsal body wall, and 

 the liver/ inclosed within the transverse septum d. E, later stage of mammals, showing union of the 

 coelomic fold j with the transverse septum d, the two together forming the diaphragm which separates 

 the pleural cavity k from the peritoneal cavity m; the liver has constricted from the main part of the 

 transverse septum, the constriction becoming the coronary ligament i. a, pericardial cavity; b, heart; 

 c, parietal pericardium or pericardial sac; d, transverse septum; e, serosa of the liver, this being a part 

 of the transverse septum originally; /, liver; g, pleuroperitoneal cavity; h, lung; i, coronary ligament 

 of the liver; _;', coelomic fold which forms part of the diaphragm; k, pleural cavity; /, pleuropericardial 

 membrane or anterior continuation of the transverse septum; in, peritoneal cavity. 



both entodermal and mesodermal elements, the latter predominating. The anterior and 

 posterior ends of the digestive tube are formed by the invagination of the surface ectoderm, 

 that at the anterior end which becomes the lining of the mouth cavity being called the slomo- 

 daeum, that at the posterior end, lining the anus, the proctodaeum. 



