LATER DEVELOPMENT OF CHICK AND RABBIT 415 



KB. 



The adult condition is reached as a result of the disappearance of the 

 whole fifth arch, the proximal parts of the first and second arches, 



and certain of the connecting 

 vessels. The presence of the 

 six arterial arches passing 

 round the pharynx in relation 

 to rudimentary gill clefts is 

 characteristic of all vertebrates 

 from Amphibia up, and recalls 

 the condition that obtains 

 permanently in the adult fish. 

 In all these higher groups, 

 too, the same arches persist 

 throughout life, and practically 

 the same portions are lost or 

 cease to function. 



The first embryonal veins to 

 appear, during the second day, 



or.pl 



A te. 



~"'fr. str. 



FIG. 145. Chick, twelve pairs somites (33 hours). From Lillie. 



H.B., hind-brain ; op.Ves., optic vesicle ; M.B., mid-brain ; F., fore-gut ; Ht ., heart ; 

 V .o.m., omphalo-mesenteric vein ; sa., 512., second and twelfth somites ; Ao., aorta. ; pr.str., 

 primitive streak; or.pl., oral plate; A.C.S., anterior cerebral suture; v.Ao., ventral aorta; 

 ai.p., anterior intestinal portal. 



are the anterior cardinal veins, a pair of vessels running back along 

 the ventro-lateral walls of the brain, each receiving an external jugular 

 vein from the floor of the pharynx. Proximally they enlarge and 



