132 EARLY EMBRYOLOGY OF THE CHICK 



the outset of its development the embryo must, therefore, 

 establish organs for the digestion and absorption of food, the 

 securing of oxygen, and the ehmination of waste products. 

 These organs serve the embryo but temporarily and are dif- 

 ferent in structure and in location from the organs which carry 

 out the corresponding functions in the adult, their nature and 

 location depending on the exigencies of the embryo's living 

 conditions. 



The main channels of the circulation in young embryos lead 

 to and from their temporary organs of digestion and absorption, 

 respiration, and excretion. The arrangement of the main 

 vessels characteristic of the adult appears only as the organs 

 characteristic of the adult develop. The changes by which the 

 circulatory system acquires its adult arrangement are of neces- 

 sity gradual. Any changes which were sufficiently abrupt to 

 interfere with' the circulation would result in disaster for the 

 embryo. Even slight curtailment of the normal blood supply 

 to any region would cause its growth to cease; any marked local 

 decrease in the circulation would result in local atrophy or 

 malformation; complete interruption of any important circula- 

 tory channel, even for a short time, would inevitably mean the 

 death of the embryo. Consequently the arrangement of 

 vessels characteristic of the embryo persists during the forma- 

 tion of the adult organs, and becomes altered only gradually as 

 the adult organs and the vessels associated with them become 

 ready to function. 



If the various circulatory channels of young chick embryos 

 are considered in the Ught of their functions, the differences 

 between the embryonic and the adult circulations should not 

 be troublesome. The circulation of young chick embryos in- 

 volves three main arcs of which the heart is the common center 

 and pumping station. One of these circulatory arcs, the vitel- 

 line, carries blood to the yolk-sac where food materials are 

 absorbed and then returns the food-laden blood to the heart for 

 distribution within the embryo. Another arc carries blood to 

 and from the allantois. The distal portion of the allantois lies 

 close beneath the egg shell and the blood circulating in the 

 allantoic vessels is thereby brought into a location where inter- 

 change of gases can be carried on with the air which penetrates 

 the shell (Fig. 30, C and D). It is in the allantoic circulation 



