THE EARLY EMBRYOLOGY OF THE MOUSE 51 



the blood islands. At this stage actual blood channels have not appeared in 

 most of the blood islands, but when these are established, a capillary network 

 is formed encircling the yolk-sac. Blood is collected from this network 

 anteriorly by the paired, omphalomesenteric veins which convey it back to 

 the heart. When the heart starts beating, this system of blood vessels 

 provides a generous circulation through the yolk-sac which serves at this 

 time as the principal organ for the procurement of food from the mother. 



Change in shape of the yolk-sac. — When the embryo starts turning, the 

 yolk-sac and ectoplacental cone form a slightly flattened sphere (Fig. 19C). 

 When turning has been completed, these bounding structures of the embryo 

 shortly assume the form of a slightly saucered-out hemisphere (Fig. 20B). 

 The ectoplacental cone becomes flattened and then dorsally concave, and the 

 yolk-sac adjacent to the cone pushes outward into the porous, blood-filled 

 vascular zone of the decidua. The embryo meantime, still attached to the 

 yolk-sac by the walls of the mid-gut, tips over so that it lies with its left side 

 adjacent to the yolk-sac, its right side facing the placenta. 



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