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 he 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 
