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 



A. A 



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); A, heart; a, b, its two chambers; c, aortic arches; m, aorta; i, 

 liver ; p, allantois. 



