TISSUE-DIFFERENTIATION 121 



functionless. This is much the same improvement 

 that would be effected in the transport of grain to 

 ships if, instead of unloading the sacks of wheat from 

 the cars to the dock and then again to the ship, an 

 arrangement were made whereby the grain could be 

 poured directly from the cars into the hold. 



In the vertebrates a comparable relation of blood 

 system and excretory tubules is found. Instead of 

 being distributed throughout the body, these tubules 

 are concentrated into a single organ, the kidney; 

 the blood-vessels supplying the kidney develop tiny 

 knots of capillaries called glomeruli (Latin, glomeru- 

 lus, " little ball "), which afford a very large surface 

 in a small space and thus permit the diffusion of the 

 maximum of waste substances in a minimum of 

 time. In the higher vertebrates there is no trace 

 of the openings of these tubules into the body-cavity. 

 In lower forms, however, such as the frog, they may 

 be observed on the surface of the kidney although 

 they are functionless. 



II. DIFFERENTIATION IN PLANTS 



Plants differ strikingly from animals in the 

 emphasis which evolution has laid upon various 

 functions common to both. Animals are, for the 

 most part, actively moving creatures, seeking food 

 in various places, and consequently endowed with 

 elaborate systems of differentiated protoplasm in 

 the form of muscles and sense-organs. In plants, on 

 the other hand, the assimilative ("vegetative") 

 function is predominant, and the manifold differ- 



