DEVELOPMENT OF THE VASCULAR SYSTEM 



507 



anterior wall of the umbilical orifice are now continuous \vith the posterior ends of 

 the anterior ventral aorta. With the formation of the tail-fold the posterior parts 

 of the primitive aortae are carried forward in a ventral direction to form the pos- 

 terior ventral aortse and primary caudal arches. 1 In the pericardial region the two 

 primitive aortse grow together, and fuse to form a single tubular heart (Fig. 460), 

 the posterior end of which receives the two vitelline veins, while from its anterior 

 end the two anterior ventral aortse emerge. 2 The first cephalic arches pass through 

 the mandibular arches, and behind them five additional pairs subsequently develop, 

 so that altogether six pairs of aortic arches are formed; the fifth arches are very 

 transitory vessels connecting the ventral aortas with the dorsal ends of the sixth 

 arches. By the rhythmical contraction of the tubular heart the blood is forced 

 through the aortse and bloodvessels of the vascular area, from which it is returned 

 to the heart by the vitelline veins. This constitutes the vitelline circulation (Fig. 

 459), and by means of it nutri- 

 ment is absorbed from the yolk 

 (vitellus.) 



The vitelline veins at first 

 open separately into the poste- 

 rior end of the tubular heart, but 

 after a time their terminal por- 

 tions fuse to form a single ves- 

 sel. The vitelline veins ulti- 

 mately drain the blood from the 

 digestive tube, and are modified 

 to form the portal vein. This is 

 caused by the growth of the liver, 

 which interrupts their direct 

 continuity with the heart; and 

 the blood returned by them cir- 

 culates through the liver before 

 reaching the heart. 



With the atrophy of the yolk- 

 sac the vitelline circulation di- 

 minishes and ultimately ceases, 

 while an increasing amount of 

 blood is carried through the um- 

 bilical arteries to the villi of the 

 chorion. Subsequently, as the 

 non-placental chorionic villi atro- 

 phy, their vessels disappear; and 



Vio nmKili^ol oT.f FlG - 458 - Diagram of the vascular channelsin a human embryo 



lie Umbilical arteries COn- of the second week. (After Eternod.) The red lines are the dorsal 



flip wVinlp rvf -rlipir r>rm-ronte a rtae continued into the umbilical arteries. The red dotted linea 



Hie WIK ItS are the ventral aorta, and the blue dotted lines the vitelline veins. 



to the placenta, whence it is re- 

 turned to the heart by the umbilical veins. In this manner the placental circu- 

 lation is established, and by means of it nutritive materials are absorbed from, 

 and waste products given up to the maternal blood. 



The umbilical veins, like the vitelline, .undergo interruption in the developing 

 liver, and the blood returned by them passes through this organ before reaching 

 the heart. Ultimately the right umbilical vein shrivels up and disappears. 



During the occurrence of these changes great alterations take place in the 

 primitive heart and bloodvessels. 



Neurenteric 

 canal 



AUantoic 

 diverticulum 



Body-stalk 





1 Young and Robinson, Journal of Anatomy and Physiology, vol. xxxii. 



1 In most fishes and in the amphibia the heart originates as a single median tube. 



