STRUCTURES AND ORGANS. 31 



it very quickly in both directions. And, indeed, a 

 solution of substances adapted for the nutrition of 

 the tissues is continually flowing through the wall of 

 the vessels towards the bioplasm of the several tissues, 

 while substances, resulting from the decay of tissues, 

 also dissolved, flow in an opposite direction from the 

 tissues through the vascular wall into the blood. The 

 blood is driven through the vessels by the pump- 

 like action of the heart into the interior of which 

 the vessels open. (Page 24.) 



4 1 ?. Structures and Organs. As the embryo in- 

 creases in size and advances in development strik- 

 ing differences in character become manifest in diffe- 

 rent parts. Peculiarities of structure are observable, 

 and the different structures perform very different 

 offices. The vessels in which blood circulates are, 

 however, distributed to them all, but the composition 

 of the blood is altered both as regards what it gives 

 up and what it receives, as it traverses the vessels 

 distributed to the several parts. 



48. Alimentary Canal. In vertebrate animals an 

 important tubs is developed which is set apart for the 

 reception and preparation of the food, and although 

 this is not required during embryonic life, it must be 

 formed so that it may be ready to do its work to 

 perform its highly important function when the newly 

 formed organism comes into the world. Its charac- 

 teristics in every stage through which it passes during 

 its construction prove to us that the form it was ulti- 

 mately to assume and the work it was to perform 

 were, as it were, anticipated from the first. For 

 months during its gradual formation it fulfilled no 

 useful purpose whatever, and yet change succeeded 

 change in pre-arranged order, until at last highly com- 

 plex structures, adapted for certain definite purposes 

 only, were evolved. The formation of these is com- 

 plete, and they are ready to discharge their function 

 at the time when their activity becomes necessary to 



