STRUCTURE OF FOUR-DAY CHICKS I3I 



V. The Circulatory System 



The Functional Significance of the Embryonic Circulation. 



The arrangement of the embryonic circulation is difficult to 

 understand only when its functional significance is overlooked. 

 In the embryo as in the adult the main circulatory channels 

 lead to and from the centers of metaboHc activity. The circu- 

 lating blood carries material from the organs of digestion and 

 absorption to remote parts of the body; oxygen to all parts 

 of the body from the organs which are specially constructed 

 to take up oxygen from the surrounding medium; and waste 

 materials from the places of their liberation, to the organs 

 through which they are eHminated. The differences between 

 the course of the circulation in the embryo and in the adult are 

 due to the fact that their centers of metaboHc activity are 

 differently located. 



The organs which in the adult carry out such functions as 

 digestion and absorption, respiration, and excretion are ex- 

 tremely complex and highly differentiated structures. They 

 are for this reason slow to attain their definitive condition and 

 do not become functional until toward the close of embryonic 

 Hfe. Moreover the conditions by which the developing adult 

 organs are surrounded during embryonic life are in some in- 

 stances an absolute bar to their becoming functional were they 

 sufficiently developed so to do. Suppose the lungs, for example, 

 were fully formed at an early stage of development. The fact 

 that the chick embryo is living submerged in the amniotic fluid 

 would render them as incapable of functioning as the lungs of a 

 man under water. Were the embryo dependent on the es- 

 tabhshment of the organs which carry on metaboUsm in the 

 adult, development would be at an impasse. To develop, the 

 embryo must have not only the raw food material supplied it 

 by the mother in the form of yolk, it must have a means of 

 digesting the yolk, absorbing it, and carrying it to the places 

 where it can be utilized. The utiHzation of food material to 

 produce the energy expressed in growth processes depends on 

 presence of oxygen. For growth there must be a means of 

 securing oxygen and carrying it, as well as food, to all parts of 

 the body. Nor can continued growth go on unless the waste 

 products Hberated by the growing tissues are eliminated. At 



