CH. I.] 



INTRODUCTORY 



Space con- 

 taining 

 liquid. 



Protoplasm. 



Nucleus. 



Cell-wall. 



FIG. 1. Vegetable cells. 



is seen in the epithelial tissues ; but in other tissues, particularly the 

 connective tissues which are not so eminently living as the rest, the 

 amount of cement or intercellular material is much greater, and in 

 this it is that the fibres are developed that 

 confer the necessary strength upon these 

 binding tissues. 



If, instead of going to the adult animal, 

 we look at the animal in its earliest stage of 

 development, the ovum, we find that it con- 

 sists of a single little mass of living material, 

 a single cell. As development progresses it 

 becomes an adherent mass of cells. In the 

 later stages of development various tissues 

 become differentiated from each other by the 

 cells becoming grouped in different ways, by 

 alterations in the shape of the cells, by de- 

 position of intercellular matter between the 

 cells, and by chemical changes in the living 

 matter of the cells themselves. Thus in 

 some situations the cells are grouped into 

 the various epithelial linings ; in others the 

 cells become elongated and form muscular fibres ; and in others, as 

 in the connective tissues, there is a preponderating amount of inter- 

 cellular 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 be- 

 come 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 eel) 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. 

 Bub the animal cell is different ; as a rule, it has no cell- wall, and 

 no vacuoles. It is just a little naked lump of living material. This 

 living material is jelly-like in consistency, possessing the power of 

 movement, and the name protoplasm has been bestowed on it. 



FIG. 2. Animal cell consisting 

 of protoplasm containing a 

 nucleus. 



