60 



INTRODUCTION TO EVOLUTION 



broinchioil grooves 



heoirt 



oirm 



iver 



umbilical 

 cord 



chorionic villi 



placenta 



FIG. 4.10. Human enbryo at the end of the first month (7 mm.). (Mainly after Gilbert, 

 Biography of the Unborn, The V/illiams & Wilkins Co., 1939.) 



the inside of the body, in the wall of the digestive tract in the region called 

 the pharynx, a corresponding series of pharyngeal or gill pouches de- 

 velops. In fish embryos the grooves on the outside finally meet the cor- 

 responding pouches from the inside. A perforation then forms, convert- 

 ing the grooves into slits or clefts — openings directly from the pharynx to 

 the exterior of the body. Typically five such gill clefts develop on each 

 side of the head of fishes. 



What is the function of the gill clefts in fishes? Fig. 4.11 presents dia- 

 grams of the head of a shark with portions of the surface cut away so that 

 mouth, pharynx, and connected structures may be seen. As the fish swims, 

 water is taken into the mouth and passes to the exterior through the gill 

 clefts (note the arrows). As it passes through the clefts the water bathes 

 the gills lining the walls of the clefts. The gills are soft fleshy structures 

 with surfaces richly supplied with capillaries of the blood system. Since the 

 blood in the capillaries is separated from the water passing through the 

 clefts by only a thin membrane, means is provided for the taking on by 

 the blood of oxygen from the water, and for the giving up to the water of 



