. Chordate anatomy. Chordata; Anatomy, Comparative. REPRODUCTION 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 tiss
. Chordate anatomy. Chordata; Anatomy, Comparative. REPRODUCTION 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, corr
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