228 THE DEVELOPMENT OF THE CHICK 



lation in the yolk-sac 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 yolk-sac; anastomosis of vessels 

 of the yolk-sac and allantois. 



The changes can be followed only in outline. The earliest 

 condition has been described in Chapters IV and V. Fig. 133 

 shows a condition intermediate between stages 1 and 2 above. 

 The network is entirely arterial, except towards the anterior 

 end, i.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 aortse 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 



