394 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



so that the ventral aorta is resolved into three distinct vessels forming 

 portions of the common pulmonary artery and of the right and left 

 systemic aortae respectively. 



The Aortic Arches and their Derivatives. — From the ventral 

 aorta there are given off on each side a series of half-hoop-shaped 

 aortic arches which pass in a dorsal direction, between successive 

 visceral clefts, to open eventually into the dorsal aorta. In the region 

 where it receives the aortic arches the dorsal aorta is frequently 

 paired (forming the aortic roots) either temporarily or throughout 

 life. This assumption of the paired condition may not improbably 

 be of similar significance to that of the ventral aorta i.e. have to do 

 merely with the economizing of material. In any case the precise 

 extent of the paired condition does not appear to be of any great 

 morphological importance. 



As regards the aortic arches themselves the following general 

 features are to be noted : (1) that they develop in order of position 

 from before backwards in agreement with the general principle of 

 development of the vertebrate body and (2) that individual arches 

 tend to become reduced in size in correlation with diminution of 

 functional activity. Thus the mandibular and hyoid arches having 

 lost or at least greatly diminished their respiratory activity even in 

 the lower fishes we find a corresponding disappearance or reduction 

 of their aortic arches. 



An important point to notice is that where the particular 

 visceral arch carries a true external gill (Crossopterygians, Zepido- 

 siren and Protopterus, Urodele and some other Amphibians) the 

 aortic arch passes out as a loop into the external gill. The aortic 

 arch is in fact in these forms during early stages, before the gill- 

 clefts are perforated, the vessel of the external gill. Such relations 

 on the part of vessels of the fundamental morphological importance 

 of the aortic arches are not to be dismissed lightly as modern adap- 

 tive modifications. They appear to indicate that the archaic 

 function of the aortic arch was to supply the external gill with 

 blood. 



As the external gill ceases to function a short circuit is formed 

 at its base, through which the blood passes directly to the dorsal 

 part of the arch without traversing the external gill. In Urodeles, 

 according to Maurer, the short-circuiting vessel sprouts downwards 

 from the dorsal limb of the arch but in Lepidosiren Robertson finds 

 it arising simply by the enlargement of pre-existing chinks. Thus, 

 the definitive aortic arch, in those cases in which an external gill is 

 for a time present, includes a portion secondarily intercalated in its 

 course and derived from the short-circuiting vessel. 



In the typical fishes, where respiration is carried on by the wall 

 of the gill-cleft, there becomes intercalated in the course of the aortic 

 arch a respiratory network of capillaries, so that the arch is divided 

 into a distinct ventral (afferent) and dorsal (efferent) portion. In 

 such a fish as Lepidosiren, where the respiratory activity of the gills 



