THE CIRCULATORY SYSTEM 



603 



branches of each side then unite to form two large vessels which extend 

 through the tail, one of them above and the other below the myotomes. 



The walls disappear in the network that has grown out from the 

 lymphatic vessels to form the large subcutaneous lymph sacs already 

 noticed in the dissection of the adult frog. 



The thoracic ducts extend posteriorly from the "lymph hearts" and 

 are probably outgrowths from them. They lie between the dorsal aorta 

 and the posterior cardinal veins. 



The posterior lymph hearts (Fig. 347) (one to three pairs in num- 

 ber) develop from the intersegmental vein just as did the anterior 

 hearts, but their development is postponed until the hind-legs appear. 



Fig. 348. 



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 pleuro- 

 peritoneal 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, /, enclosed 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 

 pleura! cavity k from the peritoneal cavity, m; the liver has constricted from the 

 main part of the transverse septum, the constriction becoming the coronary liga- 

 ment, i, a, pericardial cavity ; 6, 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 liga- 

 ment of the liver ; j, coelomic fold which forms part of the diaphragm ; k, pleural 

 cavity ; I, pleuropericardial membrane or anterior continuation of the transverse 

 septum; m, peritoneal cavity. (From Hyman's "A Laboratory Manual for Com- 

 parative Vertebrate Anatomy," by permission of the Chicago University Press.) 



