GENERAL ANATOMY OR HISTOLOGY. 



THE ANIMAL CELL (Fig. 596). 



ALL the tissues and organs of which the body is composed were originally 

 developed from a microscopic body (the ovum), consisting of a soft gelatinous 

 granular material enclosed in a membrane, and containing a vesicle, or small 

 spherical body, inside which are one or more solid spots. This may be regarded 

 as a perfect cell. Moreover, all the solid tissues can be shown to consist largely 

 of similar bodies or cells, differing, it is true, in external form, but essentially 

 similar to an ovum. 



In the higher organisms all such cells may be defined as " nucleated masses of 

 protoplasm of microscopic size." The two essentials, therefore, of an animal 



Cell wall. 



Attraction sphere (centra- 

 sphere) enclosing two 

 centrosomes. 



Nuclear membrane. 



Nucleus. 

 Chromatin network. 



fet-knot of chromatin form- 

 ing a pseudo-nucleus. 



Vacuoles. 

 Fat granules. 



FIG. 596. Diagram of a cell. (Modified from Wilson.) 



cell in the higher organisms are, the presence of a soft gelatinous granular 

 material, similar to that found in the ovum, and which is usually styled proto- 

 plasm ; and a small spherical body imbedded in it, and termed a nucleus; 1 the 

 remaining constituents of the ovum viz., its limiting membrane and the solid 

 spot contained in the nucleus, called the nucleolus are not considered essential 

 to the cell, and in fact many cells exist without them. 



Protoplasm (cytoplasm) is a material probably of variable constitution, but 

 yielding to the chemist on its disintegration bodies chiefly of proteid nature. 

 Lecithin and cholesterin are constantly found in it, as well as inorganic salts, 

 chief among which are the phosphates and chlorides of the alkali metals and cal- 

 cium. It is of a semifluid, viscid consistence, and appears either as a hyaline sub- 

 stance, homogeneous and clear, or else it exhibits a granular appearance. This gran- 



1 In certain lower forms of life masses of protoplasm without a nucleus have been described by 

 Huxley and others as cells. 



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