XIV ELEMENTS OF VERTEBRATE EMBRYOLOGY 465 



artery (Fig. 196, A, d.c). From the hinder portion of the dorsal aorta 

 which becomes again paired there passes straight out on each side a 

 large vitelline artery (y.a) to supply the network of the vascular 

 area. 



Into the atrial portion of the heart there opens on each side a short 

 vessel {d.C), formed at its dorsal end by the meeting of two vessels one 

 of which comes from the region of the kidney and the other from the head. 

 Of these the short vessel is clearly the duct of Cuvier, that coming from 



CLCV. 



Diagram showing ttie main parts of tiie vascular system as seen in a Fowl embryo during the 

 third day (A) and the fifth day (B). a.a, allantoic artery ; a.c.v, anterior cardinal vein ; af, atrium ; 

 a.Vt allantoic vein ; d.Q, duct of Cuvier ; rf.c, dorsal carotid ; il.a, iliac artery ; ^.a, pulmonary artery • 

 p.c.v, posterior cardinal vein; p.v.c, posterior vena cava; V, Ventricle; v.A^ ventral aorta; 

 v.a, vitelline artery ; v.c, ventral carotid ; u.t/, vitelline vein ; I-VI, aortic arches. 



the kidney (;p.c.v) the posterior cardinal vein, and that from the head 

 (a.c.v) the anterior cardinal. 



The vitelline veins are present as before (p.v) and form by their fusion 

 the hinder end of the cardiac tube or heart. 



It is naturally of great interest that in this early stage in the develop- 

 ment of a Bird — one of the most highly evolved vertebrates — the heart 

 should be in the extremely primitive form of a simple contractile tube, 

 that there should be present typical aortic arches and typical gill clefts 

 though these are never meant to carry out their primitive function, and 

 finally that the main veins in the body should be precisely the same, as 

 those of a typical fish. 



2 H 



