786 



DEVELOPMENT OF THE VASCULAR SYSTEM. 



increases, and the venous is doubled back upon the arterial end. The 

 tube also becomes divided by two slight constrictions into three 



Fig. 585. — Human Embryos 



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 Coste) ; «, frontal plate ; 

 h, protovertebrse, on which, 

 the primitive aortiB 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 ; h, 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 ; 8, ventricular part of 

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

 cardium is seen. 



portions, of which that originally posterior and receiving the veins is 

 the widest, and constitutes the primitive auricle ; the middle one, next 



^\ ;a" 



Fig. 586.— Diagrammatic Outlines of the Heart and First Arterial Vessels 

 OP THE Embryo, as seen from the Abdominal Surface. 



A, at a period corresponding to the 36th or SStli 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 

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



