3IO 



COMPAKATIVE MORPHOLOGY OF VERTEBRATES 



tion is developed, but as the yolk sac contains no yolk, it is of minor importance 

 and is soon lost. 



In the amniotes an outgrowth, the allantois (p. 285), arises as a diverticulum 

 from the hinder end of the alimentary canal, increases in extent, growing down- 

 ward and carrying the ventral body wall before it. Branches of the hypo- 

 gastric arteries, known as the allantoic arteries, extend into it (fig. 329) and 

 are connected by capillaries with umbilical veins which arise from the sub- 

 intestinal vein behind the vitelline veins. There thus is formed an allantoic 

 circulation which is both respiratory and nutritive in character. In the reptiles 

 both of the umbilical veins persist through the foetal life (only one shown in 

 fig. 308), but in birds and mammals one aborts, leaving the other as the efferent 

 vessel of the allantois. With the end of foetal life (at hatching or at birth) both 

 the vitelline and the allantoic circulations disappear, leaving only inconspicuous 

 rudiments. 



Fig. 330. — Developing anterior veins of Scyllium embryo, 26 mm. long; after 

 Grosser. b^~^ veins of the visceral arches; cd, Cuvierian duct; A, vein of hyoidarch; 

 ij, inferior jugular; m, vein of mandibular arch; 05, orbital sinus; «;, segmental veins; 

 vca, superior jugular; vcp, postcardinal; III-X, cranial nerves; 2-8, spinal nerves. 



The entrance of the Cuvierian ducts into the heart was mentioned 

 on page 294, These ducts are a pair of transverse vessels which enter 

 the sinus venosus, one from either side, and, together with the hepatic 

 veins, mark the posterior limit of the heart. Each develops outside 

 of the somatic wall of the hypomere and extends dorsally until it 

 reaches the level of the top of the ccelom (fig. 327). In this course, 

 in the fishes, each receives an inferior j ugular vein which comes from 

 the hea d, bringing back blood from the muscles of the lateraTanH 

 ventral branchial regions. At its dorsal end each Cuvierian duct 

 di vide s into _the_two card inal v5ns, a supeiiQr_jiigiUar_(anterior 

 cardinal) and a po stcardinal V jgin (fig. 330), which belong to the 

 dorsal half of the bddy. The superior jugular comes from the head, 



