EVOLUTION AS SEEN IN EMBRYONIC DEVELOPMENT 63 



forms an arrangement of arteries as though it were getting ready to sup- 

 ply blood to such a series of gills. At this stage in its development the hu- 

 man heart has one auricle and one ventricle, as does the heart of a fish 

 embryo or adult. Running forward from the ventricle is a single ventral 

 aorta (Figs. 4.12; 4.13A); this gives rise to several aortic arches which 



brain 



notochord 

 ear 



aortic arches 



\/olk sac 



umbilical cord 



pharyngeal 

 pouches 



ventrofl aorta 



ung bud 

 dorsal aorta 



spinal cord 



stomach 



hind gut 

 allantois 



FIG. 4.12. Human embryo of 4 mm. dissected to show digestive tract and aortic arches. 

 (After Arey, Developmental Anatomy, W. B. Saunders Co., 1 947.) 



pass between the successive pharyngeal pouches. The aortic arches are 

 continuous vessels, not interrupted by series of capillaries as they would be 

 if gills actually appeared. As in the fishes, the dorsal ends of the aortic 

 arches connect to the dorsal aorta. Fig. 4.I3A shows a human heart of 

 this stage removed from the body, together with the accompanying ventral 

 aorta and the bases of some of the aortic arches. Six pairs of aortic arches 

 appear, although not all are fully developed at any one time. 



The fidelity with which the human embryo, in common with embryos of 

 other mammals, and of birds, reptiles, and amphibians, repeats the fish- 



