EVOLUTION AS SEEN IN EMBRYONIC DEVELOPMENT 



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external gill sli+v 

 gil 



internal gill slit 



external 

 gill slit 



heart 



ventroil aorta 

 gill cleft 



FIG. 4.11. Respiratory mechanism (upper diagram), and heart and aortic 

 arches (lower diagram) of a fish (shark). A "window" has been cut into the 

 pharynx. In upper diagram arrows indicate paths of water currents. In lower 

 diagram arrows indicate direction of blood flow. 



carbon dioxide carried by the blood. This is the mechanism by which the 

 fish breathes; in other words, gill slits or clefts are part of the respiratory 

 mechanism of fishes. 



Why do the embryos of other vertebrates, such as reptiles, birds, and 



