PRIMARY FUNCTIONS OF THE ORGANISM 27 



another point of view we may consider the protozoan 

 individual to be comparable to the metazoan indi- 

 vidual, with the difference that for purposes of utility 

 the body of the latter is subdivided into a great 

 number of dynamic centers (cells), whereas that of the 

 former is not so divided. 



FIG. 7. A white blood corpuscle (leucocyte) of the frog sketched 

 at frequent intervals ; from n to m the temperature was gradually 

 raised, then lowered (n to p).^(From Hertwig, after Engelmann.) 



In the metazoan body, however, some of the 

 cells live a relatively free and independent existence, 

 and we will begin our study of the phenomena of 

 cell specialization with such a cell. 



In the blood stream of most animals are to be found 



