EMBRYOLOGY OF THE CHICK 



diverticula of the omphalomesenteric veins) extend out over the yolk, 

 constantly increasing as to both absolute numbers and as to branches, 

 as the yolk-sac spreads over the yolk. 



The allantoic arteries are also called umbilical arteries. They are 

 what will later be known as hypogastric arteries. In birds and reptiles 

 five vessels, three arteries (one omphalomesenteric and two allantoic;, 

 and two veins (one vitelline, really omphalomesenteric, and one allan- 

 toic), connect the embryo freely through the umbilical stalk (Figs. 284, 

 297, 308). 



In mammals, where there is little or no yolk, the yolk-sac is reduced 

 or absent entirely and the omphalomesenteric and vitelline vessels dis- 

 appear very early, so that the umbilical cord or stalk contains only the 

 two allantoic arteries and one allantoic vein. 



In the dogfish and all elasmobranchs, where there is a large yolk-sac 

 but no allantois, the vitelline circulation alone is found, the allantoic 

 not being present. 



THE INTRA-EMBRYONIC CIRCULATION 



The large vessels communicating with the heart are the first ones to 

 appear in the chick embryo. At thirty-three hours the ventral aorta ex- 

 tends headward, bifurcating ventral to the pharynx to form a single pair 

 of aortic arches. This pair of arches passes dorsad around the pharynx, 

 then running tailward on the dorsal wall of the gut as the paired dorsal 

 aortae (Fig. 277). 



On the second day, as the visceral arches and clefts appear, this 

 original pair of aortic arches comes to lie in the mandibular arch. In 

 each of the visceral arches posterior to the mandibular, new aortic arches 

 are formed, which connect the ventral aortae with the dorsal aortae. 



At fifty-five hours we saw there were three pairs of these aortic 



Cuvieri ; dv, ductus venosus ; g, gut ; hi, left hepatic vein ; hr, right hepatic vein ; 



I, liver ; o, omphalo-mesenteric vein ; p, anterior intestinal portal ; pa, rudiment 

 of pancreas ; ul, left umbilical vein ; ur, right umbilical vein ; v, vitelline vein ; I, 



II, primary and secondary venous rings around the gut. (After Hochstetter. ) 

 G to J, Diagrams to show the origin of the postcaval vein and the changes 

 in the abdominal vein in amphibians and reptiles. G, elasmobranch stage. The 

 lateral abdominal veins i enter the common cardinal veins c and are not connected 

 with the renal portal veins p. H, the lateral abdominals i have joined the renal 

 portals at t posteriorly, and anteriorly pass into the liver /, where they unite with 

 the hepatic portal vein h; a new vein, the postcaval vein g, is seen growing caudad 

 from the liver /, where it arises from the hepatic veins o. I, condition in the 

 adults of urodele amphibians ; the postcaval vein g, has reached and fused with the 

 posterior cardinals e and and the subcardinals j at the point r; the two lateral 

 abdominal veins have united to form the ventral abdominal vein i which empties 

 into the hepatic portal h. J, condition in adult reptiles ; the anterior portions of 

 the posterior cardinal veins n are obliterated, leaving the postcaval vein g as 

 the sole drainage for the subcardinals j and the kidneys k; the two lateral ab- 

 dominal veins remain separate as in elasmobranchs. a, anterior cardinal vein ; b, 

 sinus venosus ; c, common cardinal vein ; d, subclavian vein ; e, posterior cardinal 

 vein; /, liver; g, postcaval vein; h, hepatic portal vein; i, lateral (or in I, ventral) 

 abdominal vein ; j, subcardinal vein ; k, kidney ; I, iliac or femoral vein ; m, caudal 

 vein ; n, obliterated part of the posterior cardinals ; o, hepatic veins ; p, renal 

 portal veins ; q, pelvic veins ; r, union of postcaval, posterior cardinals, the sub- 

 cardinals ; s, union of postcaval and subcardinals ; t, union of abdominal vein with 

 the renal portal system. (From Hyman's "A Laboratory Manual for Comparative 

 Vertebrate Anatomy," by permission of The Chicago University Press.) 



