62 THE CELL. 



Many cells are without a distinct cell membrane, another con- 

 stituent of the protoplasm. In such cells the outer layer of the 

 protoplasm is often more homogeneous and less dense than that 

 lying more centrally, which has often a more granular appearance; 

 the outer layer of the protoplasm is in such cells known as the 

 exoplasm, in contradistinction to the more granular endoplasm. 



In other cells, however, the outer layer of the cell-protoplasm 

 shows differentiation, leading to the formation of. a distinct cell-mem- 

 brane (as in fat-cells, cartilage-cells, goblet-cells, etc.). F. E. Schulze 

 has given it the name pellicula in cases where the entire cell is sur- 

 rounded by a homogeneous layer, and cuticula or cuticle where 

 only one side of the cell is supplied with the membrane (as in the 

 intestinal epithelium). It is assumed that both spongioplasm and 

 paraplasm are concerned in the formation of this membrane. 



In the protoplasm of many cells there is found a small body 

 known as the centrosome. This is usually situated near the nucleus 

 of the cell, occasionally in the nucleus. Generally, it has the appear- 

 ance of a minute granule, sometimes scarcely larger than a micro- 

 some. It is often surrounded by a small area of a granular or finely 

 reticular or radially striated cytoplasm, known as the attraction- 

 sphere or centrosphere. 



B. THE NUCLEUS. 



The second constant element of the cell is the nucleus. As a 

 rule, it is sharply defined, and in its simplest form consists of a 

 round vesicle of a complicated structure composed of several sub- 

 stances. The form of the nucleus corresponds in general to the 

 shape of the cell ; in an elongated cell, it is correspondingly long, 

 and flattened where the cell is plate-like in shape. The nucleus of 

 a wandering cell that is in the act of passing through a narrow inter- 

 cellular cleft adapts itself to the changes of form in the cell without 

 being permanently altered in shape. In other words, the nucleus is 

 soft, and can be easily distorted by any solid substances within or 

 without the protoplasm, only to resume its original form when the 

 pressure is removed. It possesses, then, a certain amount of elas- 

 ticity. Movements of certain nuclei, entirely independent of the sur- 

 rounding protoplasm, have often been observed. It is only rarely 

 that the general form of the nucleus differs materially from the 

 general form of the cell. This, however, occurs in the nuclei of 

 leucocytes and many of the giant cells of bone-marrow, which 

 are often irregular, and may even be ring-shaped. In certain arth- 

 rozoa, branching forms of nuclei occur, as also in the skin glands 

 of turtles. The proportionate size of nucleus to cell-body varies 

 greatly in different cells. Especially large nuclei are found in im- 

 mature ova, in certain epithelial cells, etc. 



The contents of the nucleus consist of a framework or reticu- 

 lum, in the meshes of which there is found a semifluid substance. 



