446 



Helen Williston Smith. 



itself through a considerable angle. This twisting is probably not 

 normal, but it permits of a very good view of the blood vessels, and 

 this embrvo has therefore been selected for illustration. The strik- 



FiG. 1. — Embryo 6 mm. long. Enlarged 17 times. 



Cor, heart ; V.o, omphalomesenteric vein ; Ao, aorta ; Y.cps, left posterior 

 cardinal vein; V.us, left umbilical vein; Y.ud, right umbilical vein; V.cas, 

 left anterior cardinal vein ; V.cpd, right posterior cardinal vein ; A.om, 

 omphalomesenteric artery; A.ns, left umbilical artery. 



ing thing about it is the extreme simplicity of its vascular system. 

 The aorta is large. It gives off a number of spicule-like vessels, 

 many of which on the ventral side unite to form the omphalo- 



