62 INTRODUCTION TO EVOLUTION 



mammals, which never breathe by means of gills, start the process of de- 

 veloping a series of gill slits? As with the somites mentioned above, this 

 seems another instance of inheritance of embryonic structure from 

 aquatic ancestors, another indication that land-dwelling vertebrates are the 

 descendants of fishes. 



It is noteworthy that in the human embryo one of the gill pouches does 

 become perforated, forming a passageway from the pharynx to the outside 

 of the head. This passageway is divided into two portions, the external ear 

 canal, leading from the outside of the head to the middle ear, and the 

 Eustachian tube, leading from the middle ear to the throat (pharynx). 

 The eardrum in the middle ear forms a thin partition between the two 

 portions of a passageway which otherwise would directly connect the 

 pharynx and the exterior as does a gill slit of a fish. 



Aortic Arches 



Mention was made above of the gills lining the walls of the gill clefts in 

 fishes and affording the means by which blood is brought in close contact 

 with the water passing through the clefts. Since gills have this function, ob- 

 viously blood vessels must be provided for transporting blood to and from 

 them. The arteries providing this transportation occur in characteristic ar- 

 rangement (Figs. 4.11; 4.13B). The heart of a fish has one auricle, which 

 receives blood from the veins. The auricle transmits the blood to the sin- 

 gle ventricle, a muscular chamber that propels the blood forward through 

 the ventral aorta. From the latter several branches pass the blood to the 

 gills fining the gill clefts. These branches are called aflFerent branchial 

 arteries; in the tissue of the gills they subdivide into a network of count- 

 less tiny capillaries. As noted above, it is while the blood is in these capil- 

 laries that it exchanges its carbon dioxide for oxygen from the water pass- 

 ing over the gills. The blood passes out of the capillaries into a series of 

 efferent branchial arteries, all of which connect to the dorsal aorta. The 

 latter has numerous branches conveying blood to aU parts and organs of 

 the body. From these parts and organs blood is returned to the auricle of 

 the heart by the veins, thus starting the cycle anew. It wiU be noted that 

 there are as many pairs of afferent and efferent branchial arteries as there 

 are gill clefts. One afferent artery plus its corresponding efferent artery 

 constitutes one aortic arch. Thus in Fig. 4.11 five aortic arches are shown. 



We have noted that in the human embryo a series of gill or pharyngeal 

 pouches forms although most of them do not perforate to form actual 

 clefts (Fig. 4.12). Needless to say, also, actual gifis never develop in the 

 walls of these pouches. Nevertheless, it is a surprising fact that the embryo 



