DEVELOPMENT. 
201 
The blood-vessels ramify in all directions through the 
yolk, making it a spongy mass, and all perform the same 
office; it is not till the fourth or fifth day that arteries 
can be distinguished from veins, by being thicker, and by 
carrying blood only from the heart . 111 
Fig. 169.—Embryo in a Hen’s Egg during the first five days: A, hypoblast; B, lower 
layer of mesoblast; C, upper layer of mesoblast and epiblast united, in the last 
figures forming the amniotic sac; D, vitelline membrane; e, thickened blasto¬ 
derm, the first rudiment of the dorsal part (in the last figure it marks the place 
of the lungs); h, heart; a, b, its two chambers; c, aortic arches; m, aorta; i, 
liver; p, allantois. 
