TISSUE-DIFFERENTIATION 1 15 



as it were, awash in the body fluids. The alimentary 

 canal occupies a relatively large part of the interior 

 space, and, in consequence, the digested food is 

 absorbed and passed on by diffusion to all parts of 

 the body. Likewise the waste-products of metabol- 

 ism find their way without difficulty to the special 

 organs that provide for their elimination. In the 

 larger animals, and particularly in those with rela- 

 tively solid tissues, with a great development of 

 muscles and an alimentary canal of small dimensions 

 compared with the whole body cavity, it Would be 

 impossible for the digested food to find its way to 

 the places where it is most needed, particularly to the 

 skeletal muscles. Appropriate to the need for trans- 

 porting such substances long distances from the 

 alimentary canal and also of transporting the meta- 

 bolic waste-products from the seat of their produc- 

 tion to the excretory organs, there is a system of 

 tubes, the circulatory system, which performs the 

 same function carried out by a railroad system in a 

 thickly settled community; that is, it transports 

 substances from the region where they are produced 

 to the region where they are utilized. The medium 

 of transportation in the case of the organism is the 

 blood. Thus, not only is the digested food carried 

 from the intestine to the tissues which are to be fed, 

 or to organs that serve as storehouses, but the 

 oxygen taken in in respiration is supplied to all 

 the tissues, the various hormones are transported 

 from one place to another, and the waste products of 

 katabolism are drained off to the excretory organs by 



