28 



NORMAL HISTOLOGY. 



parts of the same cell. And it is probable that all protozoa, i. e. 

 unicellular animals, possess similar, though less obvious and in- 

 ternal, heterogeneity of constitution. 



The less the degree of specialization or differentiation in the 

 structure of an organism, the less highly developed is the functional 

 activity of which it is capable, and the less perfect its ability to 

 cope with possible unfavorable environment. The value to the 

 whole organism of a diversity in its parts is, therefore, unquestion- 

 able, and the higher we go in the animal kingdom, the greater we 

 find the development of this diversity, coupled with a more and 

 more perfectly adjusted co-operative interdependence of the differ- 

 ent parts of the body. 



In the protozoa the single cell does all the work of the whole 

 organism. In the multicellular animals, the metazoa, this work is 

 distributed among the component cells of the body, each of which 

 has developed an efficiency for performing its special work that 

 would be incompatible with a wider range of duties. 



It is quite impossible to find in nature any example of a cell 

 devoid of all individual peculiarities attributable to differentiation 

 or specialization. We must, therefore, study several varieties of 



FIG. 6. 



Amoeba pellucida. (Frenzel.) a, ectoplasm ; b, endoplasm ; c, nucleus ; d, nucleolus ; e, large 

 contractile vacuole ; /, incorporated foreign body ; g, g, pseudopodia. 



cell in order to gain an ideal conception of such a cell. This accom- 

 plished, we may consider those cells which occur in nature as special 

 modifications of that type. 



Perhaps the simplest cell leading an independent existence is the 

 protozoon, amoeba (Fig. 6). This animal is widely distributed in 



