DEVELOPMENT OF FOURTH DAY 523 



proving that it is the potassium solution which causes the reaction." 

 It will be remembered that it was stated in the preceding paragraphs 

 that the action of the muscles will continue for some time until external 

 conditions cause a stoppage. The removal of potassium solution from 

 the surrounding medium has nothing to do with the reaction ability in 

 the muscle cell itself, but its removal removes a factor necessary to re- 

 action, by making the medium one in which it cannot react. An exam- 

 ple may make the matter clearer. A living human being has the power 

 to move his arms and walk about. This power is retained for many 

 years. Let us suppose that we remove certain substances from the air 

 which are needed for his lungs to function. An individual breathing 

 such an atmosphere would either slowly or rapidly (depending upon 

 what gases are removed) grow less and less able to move his arms or 

 to walk, and in a short time this ability would cease entirely. In other 

 words, such an individual needs a certain kind of atmosphere for breath- 

 ing purposes, without which he cannot perform his normal functions. 

 This, however, is vastly different from saying that the constituents of 

 air are the cause of his being able to move. 



From what has been said above, all that we can say, in regard to 

 nerve-and-muscle-action, is that experiments tend to demonstrate that 

 muscle cells have the ability to act and react, and that the nerves are 

 only the connectors and impulse carriers, by which a coordination of 

 muscle cells, which are not in contact with each other, may be brought 

 about. 



In the embryo the yolk is converted into blood, and the pressure 

 of that blood as it passes through the various vessels with its greater 

 posterior and its less anterior pressure, brings about the results men- 

 tioned above. In the adult, the food that is taken in and converted into 

 blood, works on quite similar principles by continuing to produce a 

 greater posterior than an anterior pressure. 



The embryonic circulation can only be understood when it is realized 

 that it varies from the adult circulation in a manner that is accounted 

 for by the difference between embryonic and adult feeding. In the 

 embryo, due to the food coming entirely from the yolk, there is devel- 

 oped a yolk or vitelline-circulation. As the chick's lungs are non-func- 

 tional before birth, and the allantois functions as a respiratory organ, 

 there is developed an allantoic circulation, while a third type is the cir- 

 culation of the embryo itself. The vitelline and the allantoic together 

 constitute the extra-embryonic circulation. 



All food material to the embryo comes from the yolk (although the 

 yolk particles do not turn directly into blood. It is the action of the 

 entodermal cells which line the yolk-sac and pour out a secretion of 

 enzymes, which breaks down the yolk granules). It is thus seen that it 

 is the vitelline vessels which carry food into the embryo, and it is the 

 allantois which serves both as a respiratory and excretory organ (at 



