THE EVOLUTION OF LIFE 



347 



corresponding to the spaces between the gills of their 

 fish-like ancestors. The arterial blood vessels of the gill 

 region have almost precisely the arrangement found in 

 the fishes, there being a number of aortic arches corre- 

 sponding to the clefts. The arrangement is such as to 

 carry blood to a series of gills, although no actual gills are 



A 



FIG. 237. Human embryos. A, right side; B, median section; C, 

 front view; a, arches of the aorta; b, brain; e, ear vesicle; gs, gill slits; h, 

 heart; uc, umbilical cord. (After His.) 



found. Later most of the aortic arches disappear, a few 

 being more or less completely retained in the adult blood 

 system. One of the gill clefts is modified to form the 

 Eustachian tube which extends from the pharynx to the 

 middle ear. The hyoid which supports several muscles 



