228 THE DEVELOPMENT OF THE CHICK 
lation in the yolk-sae may be divided into the following stages 
(following Popoff): 
1. Indifferent network bounded peripherally by the vena 
terminalis, connected by two anterior vitelline veins with the 
heart; no arterial trunks. 
2. Origin of an arterial path in the network; the right anterior 
vitelline vein begins to degenerate. 
3. Origin of intermediate veins; the (left) posterior vein 
begins to develop. 
4. Development of collateral veins; further degeneration of 
the right anterior vein; complete formation of the posterior vein. 
5. Further branching; development of a rich venous network; 
the vena terminalis begins to degenerate. 
6. Definitive condition; development of a rich venous net- 
work in the folds or septa of the volk-sac; anastomosis of vessels 
of the yolk-sae and allantois. 
The changes can be followed only in outline. The earliest 
condition has been described in Chapters IV and VY. Fig. 133 
shows a condition intermediate between stages 1 and 2 above. 
The network is entirely arterial, except towards the anterior 
end, 7.e., the blood flows outwards away from the heart. It 
enters the vena terminalis and is returned by right and left an- 
terior vitelline veins to the heart. The beginning of arterial 
trunks in the network is indicated particularly on the left side 
(right side of the figure). The connection of the arterial network 
with the dorsal aorta is still net-like. 
Fig. 134 shows an advance of the same processes. The trunks 
of the vitelline arteries are better differentiated from the network, 
and the blood is still returned to the heart entirely by way of 
the vena terminalis and the right and left anterior vitelline veins, 
which have come in contact distally, circumscribing in their 
proximal parts the mesoderm-free area of the blastoderm. The 
beginning of the lateral vitelline veins is indicated, particularly 
on the right side (left of the figure). 
Fig. 135 represents a great advance. The vitelline arteries 
arise from the dorsal aortze as single trunks, and branch in the 
vascular network, some of them reaching as far as the vena 
terminalis. The two anterior vitelline veins have fused in front, 
and the right anterior vein is reduced in size so that most of the 
blood reaches the heart through the left anterior vein. But the 
