CH. I.] INTRODUCTORY 5 



material which may become permeated with fibres, or be the seat of 

 the deposition of calcareous salts, as in bone. Instances of chemical 

 changes in the cells themselves are seen on the surface of the body, 

 where the superficial layers of the epidermis become horny ; in the 

 mucous glands, where they become filled with mucin, and in adipose 

 tissue, where they become charged with fat. 



The term cell was first used by botanists ; in the popular sense 

 of the word a cell is a space surrounded by a wall, as the cell of a 

 prison, or the cell of a honeycomb. In the vegetable cell there is a 

 wall made of the starch-like material called cellulose ; within this is 

 the living matter, and a number of large spaces or vacuoles filled 

 with a watery fluid. The use of the term cell by botanists was 

 therefore completely justified. 



But the animal cell is different ; as a rule, it has no obvious cell- 

 wall, and vacuoles are not conspicuous. It is just a little naked 

 lump of living material. This living material is jelly-like in con- 

 sistency, possessing the power of movement, and the name proto- 

 plasm has been bestowed on it. 



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, such as amoebae, consist of one cell only; 

 the simplest plants, such as bacteria, torulaa, etc., consist of one 

 cell only. 



FIG. 2. Amoebae; unicellular animals. Fm. 3. Cells of tb.e yeast 



plant in process of bud- 

 ding ; unicellular plants. 



These 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. 



The higher animals and plants are always unicellular 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, 



