64 



INTRODUCTION TO EVOLUTION 



embryo stage of heart structure and arterial arrangement is amazing, af- 

 fording one of the most beautiful examples of embryonic recapitulation. 

 In the fish most of the aortic arches persist to provide blood circulation 

 through the gills, as previously noted; the heart remains permanently in 



oiorfic arches 



Venfroil 

 Aor+d 



auricle 



sinus venosus 



veins 



ventricle 



afferent branchial 

 arteries 



auricle 



nus venosus 



Ventral Aorta 



ventricle 



B 



FIG. 4.13. Heart and aortic arches of. A, early human embryo (cf. Fig. 4.12) and, B, 

 fish {cf. Fig. 4.1 1). Arrows indicate direction of blood flow. 



the single-auricle, single-ventricle state. The heart of the human embryo 

 soon develops two auricles and two ventricles, the extra one of each being 

 connected with a secondary circuit of circulation for the lungs. In the hu- 

 man embryo the six pairs of aortic arches have a varied fate. Three of 

 them disappear. Of the other three, one persists in connection with the 



