IV.] THE VASCULAR SYSTEM. 91 



In B are seen in the diverging folds of the splanchnopleure 

 the two vitelline veins (of) which will shortly unite to form 

 the ductus venosus. 



"IT 



TRANSVERSE SECTION OF AN EMBRYO AT THE END OF THE 

 SECOND DAY PASSING THROUGH THE REGION OF THE BULBUS 

 ARTERIOSUS. (Copied from His.) 



M. medullary canal in the region of the hind brain ; V. anterior 

 cardinal vein ; Ao. Aorta ; Ch. Notochord ; al. alimentary 

 canal ; H. Heart (bulbus arteriosus) ; Pp. Pleuroperitoneal 

 cavity; am. amnion. 



In the vascular and pellucid areas, the formation of 

 vascular channels with a subsequent differentiation 

 into arteries, capillaries and veins, is proceeding rapidly. 

 Blood-corpuscles too are being formed in considerable 

 numbers. The mottled yellow vascular area becomes 

 covered with red patches consisting of aggregations of 

 blood-corpuscles, often spoken of as blood-islands. 



Round the extreme margin of the vascular area and 

 nearly completely encircling it, is seen a thin red line, 

 the sinus or vena terminalis (Fig. 36, Sv.). This will soon 

 increase in size and importance. 



From the vascular and pellucid area several large 

 channels are seen to unite and form two large trunks, 



