100 ANIMAL PHYSIOLOGY. 
nial and spinal, bud out from the nervous centers. The ali- 
mentary canal enlarges, a fore-gut and hind-gut being formed, 
the former being divided into cesophagus, stomach, and duode- 
A 
A, 
( = 
Lp 
Fie. 118.—Diagram of circulation of yelk-sac at end of third day (Foster and Balfour). 
Blastoderm seen from below. Arteries made black. H, heart; AA, second, third, and 
fourth aortic arches; AO, dorsal aorta; L. Of. A, left vitelline artery; R. a A, right 
vitelline artery ; S. T, sinus terminalis; L. Of, left vitelline vein; R. Of, right vitelline 
vein; S. V, sinus venosus; D. C, ductus Cuvieri; S. Ca. V, superior cardinal or jugular 
vein; V. Ca, inferior cardinal vein. 
num; the latter into the large intestine and the cloaca. The 
lungs arise from the alimentary canal in front of the stomach; 
from similar diverticula from the duodenum, the liver and 
pancreas originate. Changes in the protovertebre and muscle- 
plates continue, while the Wolffian bodies are formed and the 
Wolffian duct modified. 
Up to the third day the embryo lies mouth downward, but 
now it comes to lie on its left side. See Fig. 109 with the ac- 
companying description, it being borne in mind that the view is 
from below, so that the right in the cut is the left in the em- 
