196 COMPARATIVE ZOOLOGY. 
causing the blood to flow backward and forward. The 
tube is gradually bent together, until it forms a double 
cavity, resembling the heart of a Fish. On the fourth 
day of incubation, partitions begin to grow, dividing the 
cavities into the right and left auricles and ventricles. 
A 
Fras. 163-167.—_Embryo in a Hen’s Egg during the first five days: A, mucous layer; 
B, vascular layer; C, blastoderm, or serous membrane, in the last figure forming 
the amniotic sac; D, vitelline membrane; e; thickened blastoderm, the first rudi- 
ment of the dorsal part (in the last figure it marks the place of the lungs); A, 
heart: a, b, its two chambers: ¢° branchial arteries; m, aorta; 7, liver; p, al- 
lantois. 
