198 STUDY OF PIG EMBRYOS. 



arch of the adult being formed from the left aorta. Lower down in the series 

 the two descending aortse unite to form the single median dorsal aorta. The 

 ducts of Cuvier are two enormous venous trunks which deliver the blood to the 

 heart. They lie symmetrically placed to the right and left of the oesophagus 

 and trachea. They extend from the level of the descending aortae downward and 

 inward to the level of the heart. The duct of the left side, D. C. S, is almost sym- 

 metrical with its fellow of the right side, but it has no direct communication with 

 the heart ; but by following down through the series of sections the student can 

 observe that the left duct of Cuvier connects across with the duct of the right 

 side. The right duct opens directly into the right auricle, Au. d, of the heart. 

 All of the venous blood is collected at this stage by the ducts of Cuvier, except 

 that which comes through the liver. The dorsal end of each duct of Cuvier is 

 formed by the union of the jugular or anterior cardinal vein from the head with 

 the posterior cardinal vein from the body. The opening of the right duct into 

 the auricle of the heart is guarded by two small flaps or valves. The lower part 

 of the section is occupied by the large heart lying in the pericardial chamber. 

 The body-wall, Som, or somatopleure, which forms the outer covering of this 

 chamber', is quite thin and without a trace of muscular or skeletal structures. It 

 consists of three distinct layers an external ectoderm and middle mesenchyma, 

 and the internal mesothelium. The mesothelium is a thin layer of cells which 

 persists throughout life and is known in the adult as the pericardial epithelium. 

 In the present section it is easy to follow this layer from the somatopleure past 

 the ducts of Cuvier on to the heart and completely around the outside of the 

 heart itself. Everywhere it forms the covering or boundary of the coelom of the 

 pericardium. In later stages this mesothelium will have an especial layer of 

 connective tissue close under it. The layer of connective tissue, together with 

 the mesothelium, constitutes the pericardial membrane of descriptive anatomy. 

 The essential fundamental relations of this membrane may, therefore, be easily 

 understood from the present section. From the study of the adult conditions 

 alone it is extremely difficult for the student to grasp these relations. The heart 

 is a very large organ. It consists of two auricles and a ventricle with two limbs. 

 The auricles have thin walls and are separated from one another by a very thin 

 membrane, the septum superius, S. s. The right auricle, Au. d, receives upon its 

 dorsal side the opening of the right vein or duct of Cuvier, the opening being 

 guarded by valves. Of these valves, the one toward the median line disappears, 

 but the other, toward the right of the embryo, persists to form both the Eusta- 

 chian and Thebesian valves of the adult. As stated above, the left duct of 

 Cuvier delivers its blood to the right duct, and so indirectly to the heart. The 

 ventricles of the heart are much larger than the auricles, and the left ventricular 



