CH. v.] THE VASCULAR AREA. 85 



for though it will afterwards become larger it will at the 

 same time become less definite and less important. We 

 may therefore, before we proceed, add a few words to the 

 description of it given in the last chapter. 



XCaj: 



DUGRAM OF THE CIRCULATION OP THE YoLK-SaCK AT THE END OF THE 



Third Day of Incubation. 



T. heart. A A. the stconfl, third and fourth aortic arches; the first 

 has become obliterated in its median portion, but is continued at its 

 proximal end as the external carotid, and at its distal end as the internal 

 carotid. AO. dorsal aorta. L. Of. A. left omphalo-mesaraic artery. 

 iJ.f/.^.rigbt omphalo-mesaraic artery. S. 2*. sinus terminaiis. Z. Of. left 

 omphalo-mesai-aic vein. if. Of. right omphalo-mesaraic vein. S. V. sinus 

 venosus. J). C. ductus Cuvieri. S. Ca. V. superior cardinal vein. 

 V.Ca. inferior cardinal vein. The veins are marked in outline and the arteries 

 are made black. The whole blastoderm has been removed from the egg 

 and is supposed to be viewed from below. Hence the left is seen on the 

 right, and vice versa. 



