DEVELOPMENT OF ORGANS 659 



the area opaca, where they cause the region over which the meso- 

 derm has extended to be distinguished, as the area vasculosa, 

 from the region outside it (Fig. 509). They arise by clumps of 

 mesenchyme cells hollowing out to form blood islands in which 

 some of the cells become corpuscles. Later the islands join into 

 a network and in this presently larger vessels differentiate. The 

 formation of blood vessels spreads over the area pellucida into 

 the embryo. The area vasculosa soon becomes bounded by a ring 

 vessel, the sinus terminalis. A little later there is differentiated 

 a pair of vitelline veins, which are continued into the embryo, 

 and bear thither the nutriment absorbed from the yolk. In the 

 embryo these join, as we shall see, below the throat to form the 

 rudiment of the heart ; vessels (Fig. 514, a.vit.v., p.vit.v.) drain 

 the sinus terminalis into them. When the body turns to he on 

 its left side (p. 663) the right half of this venous system dwindles. 



EMBRYONIC CIRCULATION 



The heart appears first at the beginning of the second day. 



It is formed from a pair of longitudinal tubes in the splanchnic 



mesoderm, each continuous behind with one of the two vitelline 



veins which run in from the yolk sac. As the splanchnopleure 



folds in under the gut, the tubes join from before backwards, 



the junction eventually proceeding for some distance along 



the vitelline veins. As in the frog the heart-tube is thrown 



into an S, constrictions mark out the chambers, and partitions 



separate them. At first there is a sinus venosus, but this later 



becomes merged in the right auricle. Like that of the tadpole, 



it receives a venous system with cardinal veins like that of a 



fish, and posteriorly is entered by the common trunk of the 



vitelline veins (ductus venosus) (Fig. 515, B). Later the posterior 



cardinals disappear, and the anterior part of the system models 



itself into that of the adult. The ductus venosus is joined on the 



fourth day by the allantoic vein, and a httle later by the inferior 



vena cava (Fig. 515, D). Towards the end of development the 



vitelline and allantoic veins, having no further function, dwindle 



and disappear, and the inferior vena cava becomes the great 



vein of the hinder part of the body. The portal system is developed 



from the ductus venosus behind the junction of the inferior vena 



cava. As the heart forms there appears a pair of dorsal aortae and 



then two vitelline arteries from the aortae to the yolk sac. Late 



