STRUCTURE OF FOUR-DAY CHICKS 133 



that the blood gives off its carbon dioxide and acquires a fresh 

 supply of oxygen. The allantoic circulation is also the em- 

 bryo's means of eliminating nitrogenous waste material from 

 the blood. The remaining circulatory arc is confined to the 

 body of the embryo. The intra-embryonic circulation has 

 many distributing and collecting vessels but all of them are 

 alike in function in that they bring food material to, and 

 carry waste material from, the various parts of the developing 

 body. Nowhere in their course are the vessels of the intra- 

 embryonic circulation involved in adding food material or 

 oxygen to that already contained in the blood they convey, and 

 nowhere do they free the blood from waste materials until well 

 along in development, when the nephroi become functional. 



In the heart the blood from the three circulatory arcs is 

 mingled. As it leaves the heart the mixed blood is not as rich in 

 food material as the blood coming in through the omphalo- 

 mesenteric veins, nor as free from waste materials and as rich 

 in oxygen as the blood returned over the allantoic veins. Its 

 condition of serviceability to the embryo is, however, constantly 

 maintained at a good average by the incoming vitelline and 

 allantoic blood. 



There is a tendency among stucfents who have done but 

 little work on the circulation to regard any vessel which carries 

 oxygenated blood as an artery , v arM any vessel which carries 

 blood poor in oxygen and high in carbon dioxide content as a 

 vein. This is not entirely correct even for the circulation of 

 adult mammals on which the conception is based. In com- 

 parative anatomy and especially in embryology it is far from 

 being the case. It is necessary, therefore, in dealing with the 

 circulation of the embryo to eradicate this not uncommon 

 misconception. 



The differentiation between arteries and veins which holds 

 good for all forms, both embryonic and adult, is based on the 

 structure of their walls, and on the direction of their blood flow 

 with reference to the heart. An artery is a vessel carrying 

 blood away from the heart under a relatively high fluctuating 

 pressure due to the pumping of the heart. Correlated with the 

 pressure conditions in it, its walls are heavily reinforced by 

 elastic and muscle tissue. A vein is a vessel carrying blood 

 toward the heart under relatively low and constan 



