. The development of the chick; an introduction to embryology . m-brane is also directly continuous w^ith the liver dorsally aroundthe roots of the great veins. Thus a membranous wall is estab-lished for the posterior part of the pericardium. In the secondplace the peritoneal cavity extends secondarily into the body-wall bounding the pericardium ventrally and laterally, and thussplits a membranous pericardial sac oE from the body-wall. Inthis process the liver appears to play an active role. At leastits anterior lobes occupy the peritoneal spaces thus established(Fig. 194). In the mammals, on
. The development of the chick; an introduction to embryology . m-brane is also directly continuous w^ith the liver dorsally aroundthe roots of the great veins. Thus a membranous wall is estab-lished for the posterior part of the pericardium. In the secondplace the peritoneal cavity extends secondarily into the body-wall bounding the pericardium ventrally and laterally, and thussplits a membranous pericardial sac oE from the body-wall. Inthis process the liver appears to play an active role. At leastits anterior lobes occupy the peritoneal spaces thus established(Fig. 194). In the mammals, on the other hand, it is the ex- THE BODY-CAVITIES 339 tension of the pleural cavities ventrally that splits the mem-branous pericardium from the body-wall. Derivatives of the Septum Transversum. From the precedingaccount it will be seen that the following are derivatives of theseptum transversum: (1) The posterior part of the pericardialmembrane. (2) The pleuro-pericardial membrane. (3) The liverwith its vessels and gastro-hepatic and primary ventral Fig. 194. — Photot;raph of a transverse section of an 8-day chick. abd. A. S., Abdominal air-sac. A. coel., Coeliac artery. Ao., o. m., Omphalomesenteric artery. Aiir. d., Right auricle. Cav. pc,Pericardial cavity. M. D., Miillerian duct. M. pc, Membranous pericar-dium. Msn., Mesonephros. Prv., Proventriculus. S., Septum ventricu-lorum. V. c. i., Vena cava inferior. V. h. d., Right hepatic vein. V. d.,Right ventricle. V. s., Left ventricle. (4) A small part of the heart (the sinus venosus). As regardsthe last, it should be noted that the anterior portion of the originalseptum transversum is gradually constricted from the majorposterior portion and becomes established as the sinus venosus; 340 THE DEVELOPMENT OF THE CHICK this subsequently becomes incorporated in the right auricle ofthe heart. (See Chap. XII). II. Separation of Pleural and Peritoneal Cavities; OriginOF THE Septum Pleuro-peritoneale The pleuro-pe
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