THE GENERAL PROPERTIES OF CELLS. 13 



granules, fat globules, or granular matter. All substances which coagu- 

 late proteids have the same effect on protoplasm. The vital properties 

 of protoplasm will be studied later. 



A membrane is usually present in all mature cells, though always 

 absent in embryonic forms. It may therefore be assumed that the mem- 

 brane results from a condensation of the outer layers of the cell-contents. 

 The membrane is apparent^ homogeneous, or may be porous. 



The nucleus, the size of which is generally in proportion to that of 

 the cell, and which is usually oval or spherical, is never absent in early 

 forms of active cells, though it may disappear when the cell reaches 

 maturity. In its mature stage it is generally reticulated, — that is, com- 

 posed of an investing cuticle within which the contents are arranged in 

 the form of a fibrillar net-work. The presence of a nucleus, which is 

 often difficult of recognition on account of its minute size, may be 

 demonstrated through the action of certain reagents, especially dilute 

 acids and staining fluids. Dilute acids render the protoplasm of cells 

 transparent without affecting the nucleus, which consequently becomes 

 more prominent; while staining fluids, such as carmine, hematoxylin, 

 and the anilin dyes, color the nucleus deeper in tint than the cell-contents. 

 The nucleus appears to be especially important in the reproductive func- 

 tions of cells, since when cells multiply by division the division always 

 commences in the nucleus. 



The nucleolus is simply a closer aggregation of the granules which 

 constitute the nucleus, and is very frequently absent. 



Both cell-wall and nucleus may be absent from the lowest elementary 

 organisms. 



As our conception of the structure of the higher animals and plants 

 is an association of elementary organisms invariably taking origin from 

 a single cell, our definition of such a simple organism or cell must be 

 modified so as to apply to the description of the simplest conceivable 

 organism capable of carrying on an independent existence. And as we 

 have seen that of the constituents of a typical cell but one, the cell-con- 

 tents, or protoplasm, is essential ; and as we know that there are 

 organisms capable of carrying on an independent existence in whom 

 neither cell-wall, nucleus, or nucleolus is to be detected, a cell may be 

 defined as a more or less homogeneous mass of organized material, — 

 protoplasm, — possessing development, growth, reproduction, nutrition, 

 and automatism. 



The best known of such undifferentiated forms of cell life is the 

 amceba, — one of the simplest examples of an animal organism. 



In its lowest form the amoeba (Protamceba primtiiva, Haeckel) con- 

 sists of a mass of jelly-like, structureless, albuminoid substance (proto- 

 plasm), which, so far as its chemical composition and general attributes 



