anterior end of the venous stem is increasingly converting 

 into the auricle. The route of the umbilical vein from its 

 branching in the liver to its union with the posterior hollow 

 vein Baer called the venous passage {ductus venosus) ; the 

 latter disappears in the fourth period. The common auricle 

 is further divided into two by a septum. The cardiac chambers 

 (ventricles) are already separated by the beginning of the 

 third period. The vessel going from the right ventricle is 

 the stem of the pulmonary artery, and that going from the 

 left ventricle is the stem of the aorta. The passages from 

 the vascular arch into the root of the aorta (Botallo's 

 ducts) narrow. Both anterior arches of this period remain 

 in connection with the cephalic and vertebral arteries, and 

 also with the arteries of the anterior extremities. In the 

 place of the middle arch only the Botallo's ducts remain; 

 from the external half of the last vascular arch the stem of 

 the aorta forms, and from the internal half, the right pulmo- 

 nary artery. The left root of the aorta soon is converted 

 into a thin vessel, the direct continuation of the Botallo's 

 duct of this side. "In this period," Baer concluded, 



the blood, which shares in respiration, passes 

 to the body through the umbilical vein, . . . 

 is mixed with the blood from the rest of the 

 body, and goes into the heart together with 

 blood returning from the liver. It (the blood — 

 L. B.) is divided into two streams, one of 

 which goes into the pulmonary artery, and the 

 other, the stranger, into the aorta .... 

 Respiration takes place in the urinary sac 

 .... The physiologists call such blood 

 circulation an incomplete double circle (II 7hh, 

 pp. 194-195 (147)) 



The fourth period after hatching is characterized by the 

 formation of a complete double circle of blood circulation. 

 The auricles are completely separated, and all the blood 

 from the body goes through the right half of the heart into 

 the lungs for gas exchange, and from them through the left 

 half of the heart, then into the whole body for nutrition. 

 Respiration through the urinary sac stops. The umbilical 

 arteries and veins become empty. The yolk artery becomes 

 a branch of the portal vein, and finally disappears. 



400 



