SIMPLE CELLS. 125 



entirely differently conceived. The cell is now usually 

 defined as a small semi-solid or semi-fluid (i.e. neither solid 

 nor fluid) dense body, the chemical nature of which is albu- 

 minous, and in which another small roundish body, generally 

 more solid, and also albuminous, is enclosed. An envelope 

 or membrane may exist, as is the case with most plant- 

 cells; but it may be wanting, as in most animal-cells. 

 Originally it is never present. The young cells are usually 

 roundish in form, but they afterwards vary very greatly. 

 The cells from different parts of the human body, repre- 

 sented in Figures 2-6, may be compared as examples. 



FIG. 4. Five thorny, or heckle-cells, the edges of which fit into each 

 other, from the epidermis ; one (6) is separated from the rest. 



The most essential feature in the modern conception of 

 the cells is, therefore, that the cell-body is composed of two 

 distinct parts. The one constituent part is the inner, and 

 is called the nucleus (cytoblastus) ; this is generally of a 

 round, oval, or spherical form, usually more solid, seldom 

 softer than the protoplasm, and consists of a peculiar 

 albuminous substance, the nuclein or kernel-substance ; the 

 second essential constituent part of every cell is the cell- 

 slime or cell-substance the protoplasm, or primitive slime 

 ( Urschleim of the older natural philosophers). This proto- 

 plasm, which surrounds the nucleus, also belongs, in point 

 of chemical composition, to the class of albuminous sub- 

 stances, and is a compound of carbon, containing some 



