16 PSYCHOBIOLOGY 



separated from one another by walls of a different substance; this is the 

 arrangement of the particles of an emulsion. 



The nucleus is a body which is in some cells approximately spherical, 

 but which in other cells has a variety of shapes. It is made visible, as are 

 the other details of cell structure, by ' staining ' the tissue prior to exami- 

 nation under the microscope. The various dyes used darken different por- 

 tions of the cell to different degrees. The nucleus, however, is usually 

 visible without staining, because its refractive effect on light is not the 

 same as that of the cytoplasm. 



In addition to the nucleus and the cytoplasm proper, a cell usually con- 

 tains certain other bodies. The most important of these are the centro- 

 somes, which have a function in cell reproduction, and plastids which 

 are concerned in the production of various organic compounds. Among 

 the latter are the amyloplastids, or starch-producing bodies, and the chloro- 

 plastids, or chlorophyl-producing bodies, of certain plant cells. In addi- 

 tion, almost all cells contain foreign particles, metaplasm ; these may be 

 particles of food not yet assimilated, or pigment, or oil, or water, or waste 

 products of cellular activity, etc. 



Some cells, principally in plants, are each surrounded by a cell wall, 

 which is produced by the cell. In animal cells, the outer layer of cyto- 

 plasm constitutes a cell membrane, which is not structurally distinguish- 

 able from the cytoplasm adjacent to it, but which nevertheless has certain 

 functional peculiarities. 



The nucleus is the controlling factor in the metabolic activity of the cell 

 (the breaking-down of chemical combinations, katabolism, and the build- 

 ing-up of other combinations, anabolism) , and upon it therefore depends 

 the growth and the reproduction of the cell, as well as its other vital func- 

 tions. It is nevertheless true that certain cells, when deprived of their 

 nuclei, may live for some time, and perform such functions as are gener- 

 ally ascribed to katabolic activity ; may respond by contraction, to stimula- 

 tion, or by transmitting the stimulation to other cells, etc. Cells in which 

 anabolic activity is especially important (gland cells), have significantly 

 large nuclei, and the nuclear surface is sometimes rendered relatively large 

 through the formation of branches or other irregularities of shape. 



The nucleus, when a section of tissue is stained with certain dyes, is 

 darker than the cytoplasm, and when examined under a high-power micro- 

 scope is seen to have the dye absorbed principally by a certain portion, 

 which is for this reason called chromatin. In addition to the chromatin, 

 there is in the nucleus a small rounded body, the nucleolus, which also 

 stains deeply, and a reticulum or network of fine fibers, the linin, the 

 meshes of which are filled with nuclear juice. In some cells, the chro- 



