324 



THE HEART. 



Fig. 242. DIAGRAMMATIC OUTLINES OF THE HEART AND FIRST ARTERIAL VESSELS 

 OF THE EMBRYO, AS SEEN FROM THE ABDOMINAL SURFACE. 



A, at a period corresponding to the 36th or 38th hour of incubation in the chick ; 

 B, and C, at the 48th hour of incubation ; 1, 1, primitive veins ; 2, auricular part of the 

 heart ; 3, ventricular part ; 4, aortic bulb ; 5, 5, the primitive aortic arches, and their 

 continuation as descending aorta ; these vessels are still separate in their whole extent in 

 A, but at a later period, as shown more fully in C, have coalesced into one tube in a part 

 of the dorsal region ; in B, below the upper 5, the second aortic arch is formed, and 

 farther down the dotted lines indicate the position of the succeeding arches to the number 

 of five in all ; 5', 5', the continuation of the main vessels in the body of the embryo ; 

 6, 6, the omphalo-mesenteric arteries passing out of the body of the embryo into the 

 vascular area of the germinal membrane. 



mence early in the form of a slight constriction on the outer surface, which marks 

 off the future auricles, the left being at first the smaller of the two; but the 

 auricular septum is not begun until after that of the ventricles is completed. About 



Fig. 243. Fig. 243. HUMAN EMBRYOES 



AT DIFFERENT EARLY STAGES 

 OF DEVELOPMENT, SHOWING 

 THE HEART IN ITS TUBULAR 



CONDITION. 



A, upper half of the body of 

 a human embryo of three weeks, 

 viewed from the abdominal 

 side (from Costo) ; a, frontal 

 plate ; 6, vertebrae, on which 

 the primitive aortse are lying ; 

 3, the middle of the tube of the 

 heart, below it the place of en- 

 trance of the great veins, above 

 it the aortic bulb. 



B, lateral view of a human 

 embryo more advanced than that 



last referred to, and somewhat imperfectly developed (from A. Thomson) ; a, the frontal 

 part of the head ; &, the vertebral column ; v, the wide communication of the umbilical 

 vesicle or yolk-sac with the intestine ; u, communication with the allantois or urachus ; 2, 

 auricular part of the heart connected with the veins posteriorly ; 3, ventricular part of 

 the bent tube ; 4, the aortic bulb ; near the extremities of the tube the divided peri- 

 cardium is seen. 



