INTRODUCTORY 



[CH. I. 



Somewhere in the protoplasm of all cells, generally near the middle 

 in animal cells, is a roundish structure of more solid consistency than 

 the rest of the protoplasm, called the nucleus. 



An animal cell may therefore be defined as a mass of protoplasm 

 containing a nucleus. 



The simplest animals, like the amoebae, consist of one cell only; 

 the simplest plants, like bacteria, torulae, etc., consist of one cell only. 



FIG. 3. Amoebae ; unicellular animals. 



FIG. 4. Cells of the yeast 

 plant in process of bud- 

 ding ; unicellular plants. 



Such organisms are called unicellular. In the progress of their 

 life history the cell divides into two ; and the two new cells separate 

 and become independent organisms, to repeat the process later on. 



In the case of the higher animals and plants, they are always uni- 

 cellular to start with, but on dividing and subdividing the resulting 

 cells stick together and subsequently become differentiated and altered 

 in the manner already indicated. In spite of these changes, the 

 variety of which produces the great complexity of the adult organism, 



FIG. 5. Human colourless blood-corpuscle, showing its successive changes of outline within 

 ten minutes when kept moist on a warm stage. (Schoneld.) 



there are certain cells which still retain their primitive structure ; 

 notable among these are the white corpuscles of the blood. 



It would appear at first sight an easy problem to distinguish 

 between a living thing, and one which is not living. The principal 

 signs of life are the following : 



1. Irritability ; that is the property of responding by some change 

 under the influence of an external agent or stimulus. The most obvious 

 of these changes is movement (amoeboid movement, ciliary movement, 

 muscular movement, etc.). 



2. Power of assimilation, that is, ability to convert into protoplasm 

 the nutrient material or food which is ingested. 



3. Power of growth ; this is a natural consequence of the power 

 of assimilation. 



