EMBRYO AND EMBRYONIC MEMBRANES 229 



most striking change is the transformation of part of the vascular 

 network into channels in which the blood flows towards the heart. 

 Of these there may be recognized the following: 1. Intermediate 

 veins arising from the vena terminalis at various places and 

 gradually losing themselves centrally in the vascular network. 

 2. The vascular network immediately behind the embryo has 

 assumed a venous character and likewise a large part of the 

 network immediately surrounding the embryo. 3. Lateral vitel- 

 line veins are beginning to develop from the anterior intestinal 

 portal backwards. 



Fig. 136, representing the circulation at a stage of about 40 

 somites, shows the completion of the primary circulation in the 

 yolk-sac. The vitelline arteries branch richly, and end in a 

 capillary network; very few arterial branches reach the vena 

 terminalis as such, and then only very fine ones. The vena 

 terminalis itself is relatively reduced; the lateral vitelline veins 

 have absorbed the network between themselves and the inter- 

 mediate veins, which now appear as prolongations of the lateral 

 veins. The right anterior vitelline vein has disappeared almost 

 entirely and the posterior vitelline vein is well developed, empty- 

 ing into the left lateral vein. 



The lateral vitelline arteries and veins are superposed as 

 far peripherally as the original intermediate veins, which lie 

 between the arterial trunks. Wherever there is superposition 

 of arteries and veins, the latter are superficial and the former 

 deep in position as seen from above. The figure also shows the 

 vascular network in the budding allantois, and some of the em- 

 bryonic blood-vessels. 



In the later stages of development the arteries are carried in 

 by the septa of the yolk-sac and lie near their free edges; the 

 veins, on the other hand, remain superficial in position. The 

 terminal vein becomes progressively reduced in importance up 

 to about the tenth day, and then gradually disappears as such, 

 being taken into the terminal capillaries. After the tenth day 

 the anterior and posterior vitelline veins decrease in importance 

 and finally become almost unrecognizable. The lateral veins, 

 on the other hand, increase in importance and return all of the 

 blood to the embryo. 



The rich network of venous capillaries in the septa of the 

 yolk-sac is shown in Fig. 137. It lies immediately beneath the 



