DEVELOPMENT OF THE HEART. 



Simultaneously with the appearance of the posterior process of 

 the septum, another fold arises, which is placed at the mouth of 

 the inferior vena cava, and forms the Eustachian valve. 



The aortic bulb likewise, by a projection from the inner wall 

 of the cavity, becomes divided into two canals, the anterior of 



FIG. 284. 



FIG. 285. 



FIG. 284. Development of the heart in the human embryo, from the 

 fourth to the sixth week. A. Embryo of four weeks. (Kolliker after Coste.) 

 B. Anterior, and C. posterior views of the heart of an embryo of six weeks. 

 (Kolliker after Ecker.) a. Upper limit of buccal cavity, b. Buccal cavity. 



c. Lies between the ventral ends of the second and third branchial arches. 



d. Ends of upper limbs, e. Liver. /. Intestine. 1. Superior vena cava. 

 I/. Left superior vena cava or connection between the left brachio-cephalic 

 vein and the coronary vein. \' f . Opening of inferior vena cava. 2. 2'. 

 Right and left auricles. 3. 3 X . Eight and left ventricles. 4. Aortic bulb. 



FIG. 285. Human embryo of about three weeks. (Allen Thomson.) 

 uv. Yolk sac. al. Allantois. am. Amnion. ae. Anterior extremity, pe. 

 Posterior extremity. 



which remains in continuity with the right ventricle, and the pos- 

 terior canal is continuous with the left ventricle. The anterior 

 thus becomes the pulmonary artery, and the posterior the perma- 

 nent aorta. 



The primitive circulations of a human embryo may be divided 



