. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. ^ EndoderrrK ^-Splanchnic^^v^>-,^ of Pharyn y/ Mesoderm ^S^rfi; ^Coelomic Cavity-^-- "*•- Somatic """"" \j>o^ Mesoderm ^^X^^/ | Pericardial Cavity^ Endocardial Tube. / \ Plate Dorsal / Segmentation Cavity Mesenchymal Cells Mesocardium _, . , Pericardial Cavity My oca rd i u m - Avj Ectoderm — Epicardiur !>&&&&* Endocardial _ . Ventral MesocaTdium Tubes Fig. 290. Diagrammatic cros
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. ^ EndoderrrK ^-Splanchnic^^v^>-,^ of Pharyn y/ Mesoderm ^S^rfi; ^Coelomic Cavity-^-- "*•- Somatic """"" \j>o^ Mesoderm ^^X^^/ | Pericardial Cavity^ Endocardial Tube. / \ Plate Dorsal / Segmentation Cavity Mesenchymal Cells Mesocardium _, . , Pericardial Cavity My oca rd i u m - Avj Ectoderm — Epicardiur !>&&&&* Endocardial _ . Ventral MesocaTdium Tubes Fig. 290. Diagrammatic cross sections showing the formation of the heart. In A the descending mesothelial plates have nearly met, with a number of mesenchymal cells between them. In b the plates have met ventrally; most of the mesenchymal cells have been utilized in forming a pair of endocardial tubes. In c the endocardial tubes have united; the plates have met dorsally; the ventral mesocardium has disappeared; addi- tional mesodermal cells are grouping about the single endocardial tube; and the pericardial cavity has been established. In u the dorsal meso- cardium has disappeared, and the primordium of the heart is attached to the wall of the cavity only at each end, as in Figure 291b. Three layers of the cardiac wall are now evident, namely, the endocardium, the lining layer; the myocardium, or muscle layer; and the epicardium, or visceral pericardium, the covering layer. of the heart, and a thin covering layer, the visceral pericardium. With the disappearance of the mesocardia, the heart lies free in the pericardial cavity attached only posteriorly, at the transverse septum, and anteriorly at its exit where it passes over into the ventral aorta (Fig. 291). Although the heart is at first a relatively simple tube, it soon enlarges and becomes modified. Included among the changes which occur are: con- strictions into several chambers; differential thickening of the myocardium, resulting in the establ
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Keywords: ., bookcentury1900, booksubjectanatomycomparative, booksubjectverte