vi AETEEIAL SYSTEM 405 



has disappeared in the adult, the persisting anterior portion receiv- 

 ing its blood through a new anastomotic channel (an) from the 

 third aortic arch. The posterior portion of the " external carotid " 

 or " lingual artery " of the adult Amphibian is really constituted by 

 this new development. 



The connexion of this newly developed portion of vessel with 

 arch III is just at the point where the short circuit is formed 

 between the afferent and efferent parts of the arch, and in Lung- 

 fishes (Lepidosiren — Eobertson, 1913) the blood- supply for the 

 external carotid comes to it, for a time during early stages, from the 

 dorsal end of arch III or from the aortic root, through what seems 

 to be a precocious development of this same short-circuiting channel. 

 In this case the vessel in question is at first simply continued from 

 its dorsal origin forwards into the external carotid : it is only later 

 that it communicates with the ventral or afferent end of arch III 

 so as on the one hand to form the short circuit, and on the other to 

 permit the blood to pass to the external carotid from the ventral aorta. 



In the more typical fishes the ventral carotid is not yet present. 

 The dorsal carotids of the two sides develop an anastomotic con- 

 nexion beneath the base of the skull so as to form with the aortic 

 roots a complete " cephalic circle " which shows characteristic differ- 

 ences in different Teleostean fishes (Eidewood, 1899). 



Intersegmental Aeteries of the Body-wall. — The dorsal 

 aorta gives off on its dorsal side paired arteries which run out into 

 the body wall between the myotomes. One of the most important 

 features of this series of intersegmental vessels is that for a time 

 during early stages of development they provide the blood-supply to 

 the limb rudiments. 



The main artery of the fore-limb — the subclavian artery — 

 appears during the stages in question to be simply a prolongation 

 into the limb rudiment from one of these intersegmental arteries — 

 not necessarily the same artery of the series in different types of 

 Vertebrate, or even in different developmental stages of the same 

 Vertebrate. Thus in Lacerta it is said to be the seventh inter- 

 segmental artery (van Bemmelen, Hochstetter, 1906) which becomes 

 the subclavian artery and in the Fowl the fifteenth (eighteenth or 

 nineteenth if the three or four intersegmental vessels in the head- 

 region are included — Hochstetter, 1890). In the Duck, Eabl (1907) 

 found that during the fifth day of incubation the subclavian varies 

 from the eighteenth to the twenty-first intersegmental artery and 

 that in some cases two or even three such vessels may pass out into 

 the limb rudiment at one time. 



Probably we may take it that the general principle at work is 

 this — that the limb, as it became shifted along the side of the body in 

 the course of evolution, received its blood-supply from successive inter- 

 segmental arteries as it came to be opposite to them, and that during 

 ontogeny there takes place an imperfect repetition of this process. 



The fact that the pectoral limb is supplied with blood by an 



