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 dnring the first five days, longitudinal view: 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; , 

 thickened blastoderm, 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; 

 ?, aorta ; ?', liver ; , allantois. 



