406 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



intersegmental artery raises the question whether or not this is to be 

 regarded as the primitive mode of blood-supply. For if this question 

 be answered in the affirmative we should be confronted with an 

 important point which would have to be borne in mind in all specula- 

 tions as to the evolutionary origin of the limbs of Vertebrates. As a 

 matter of fact, however, recent investigations tend to answer this 

 question in the negative. 



In the Chick (Evans, 1909) the limb rudiment in its earliest 

 stages is traversed by an irregular network of blood spaces and this 

 receives its blood-supply directly from the dorsal aorta by a number 

 of slender channels — it may be as many as ten or eleven on the 

 right side where they are commonly most numerous. These vessels 

 are scattered irregularly over an antero-posterior extent of from 

 three to five mesoderm segments and they take their origin from the 

 dorsal aorta quite independently of and considerably ventral to the 

 intersegmental arteries. As development proceeds a few of these 

 supply channels — those which happen to be most nearly inter- 

 segmental in position — become relatively larger and finally a single 

 one, at about the level of the eighteenth intersegmental artery, 

 becomes especially enlarged and carries the main stream of blood to 

 the limb rudiment while the others gradually diminish in size and 

 eventually disappear. The persisting enlarged vessel becomes the 

 subclavian artery and secondarily its origin from the aorta becomes 

 displaced in a dorsal direction until eventually it arises by a common 

 root along with the intersegmental artery of which it now appears 

 to form a branch. 



In the Duck similar observations have been made so we are 

 probably justified in stating that in the earliest stages of ontogenetic 

 development the blood-supply of the pectoral limb is not metameric, 

 and that the relation with the intersegmental artery observed in 

 slightly later stages is a secondary acquirement. 



In most vertebrates the subclavian artery which arises in the 

 manner above described (primary subclavian) persists throughout 

 life. In Birds however a cross connexion develops between the 

 primary subclavian just at the base of the limb, and the ventral end 

 of the third aortic arch. This cross connexion gradually increases in 

 size while the proximal part of the primary subclavian, arising from 

 the aortic root, becomes correspondingly reduced and eventually 

 disappears entirely. The result is that the permanent artery of the 

 fore-limb in the adult branches off, not from the dorsal aortic root but 

 from the definitive (i.e. morphologically " internal ") carotid, close to 

 its hinder end. A similar substitution takes place in Chelonians and 

 Crocodiles (Fig. 187a, C) and this is the explanation, first given by 

 Mackay (1889), of the otherwise puzzling fact that in certain Verte- 

 brates the subclavian artery passes out ventrally to the vagus nerve 

 (" secondary subclavian ") instead of dorsally as it does normally. 



The iliac artery to the hind limb has also been traced back in at 

 least Lizards and Birds to the series of intersegmental arteries but 



