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77 



tive heart. In some cases, at first two endothelial tubes are formed, 

 lying side by side, later coalescing into one. The thick muscular layer 

 (myocardium) and the outer layer (epicardiimi) of the wall of the heart, 

 also the pericardium lining the pericardial cavity, are derived from the 

 adjacent hypomeric mesoderm. The heart muscle, however, unlike that 

 of blood-vessels, is striated. 



The transverse septum, separating pericardial from abdominal 

 cavity, consists of pericardium in front and peritoneum behind, with 

 connective tissue between. The diaphragm of the mammal is not the 

 exact equivalent of the transverse septum of other vertebrates (Fig. 71). 

 That part of the coelomic space lying on the cephalic side of the diaphragm 

 is subdivided into three cavities, the pericardial and the right and left 

 pleural cavity containing the corresponding lobes of the lungs. The 



Fig. 71. — Diagrams showing the relations of the coelomic cavities (black) in fishes 

 (^4), amphibians and sauropsida (B), and mammals (C). L, liver; P, lungs; S, septum 

 transversum; D, diaphragm. In B the lungs lie in the peritoneal (or pleuroperitoneal) 

 cavity; in C they occupy special pleural subdivisions of the coelom. (From Kingsley.) 



diaphragm is muscular. Its muscle is striated and, like body-wall muscle, 

 is derived from epimere mesoderm. Strangely, however, it is mesoderm 

 which shifts backward from somites of the neck region. This accounts 

 for the innervation of the diaphragm by cervical spinal nerves. 



Head, Neck, Tail. The mesoderm of the head is less definitely seg- 

 mented than that of the trunk. The six muscles, consisting of striated 

 fibers, which effect the movements of the eyeball in its orbit are developed 

 from head mesoderm which is probably the equivalent of three somites or 

 epimeres of the trunk. There is nothing corresponding to the mesomere 

 of trunk mesoderm. 



The neck region, whether or not differentiated externally, corresponds 

 approximately to that of the embryonic pharyngeal pouches. In this 

 region the dorsal mesoderm forms epimeres which give rise to neck muscles. 

 The lateral mesoderm, remaining unsegmented,* corresponds to the 

 hypomere of the trunk. Whereas the trunk hypomere forms onl}- 



