6 HISTOLOGY 



linin fibers (Fig. i). The masses, known as chromatin knots, occur espe- 

 cially at the points of intersection in the linin meshwork. Sometimes they 

 are attached to the nuclear membrane, or so distributed over its surface 

 that it appears to consist of chromatin. It forms morphologically the 

 most important part of the nucleus. 



Certain nuclei contain one or more round bodies, which belong with 

 the chromatic elements because of their deep staining, but which are 

 chemically different from chromatin. These bodice, known as nucleoli, 

 are stained with acid or neutral dyes. They are said to be composed of 

 paranuclein, whereas chromatin is composed of nuclein. In distilled 

 water the structures formed of nuclein disappear, but those consisting of 

 paranuclein remain. The nuclei of nerve cells contain typical nucleoli 

 (Figs. 3 and 5). Sometimes a nucleolus, lodged in the nuclear reticulum, 

 is more or less covered with chromatin (Fig. 9, A), but the term should 

 not be applied to irregular knots of chromatin, even when most of the 

 chromatic material within a nucleus is gathered into one or two such 

 bodies. These are the so-called false nucleoli (pseudonucleoli). 



Every nucleus, therefore, consists of ground substance or nuclear sap, 

 a network of linin, and granules and masses of chromatin. Usually it is 

 surrounded by a membrane, and sometimes it contains a nucleolus. Most 

 cells contain a single nucleus; but occasionally a single cell contains two 

 nuclei, as is frequent in the liver, or even several nuclei, as in certain cells 

 associated with bone. Non-nucleated bodies, like the mammalian red 

 blood corpuscles, and the dead outer cells of the skin, have lost their 

 nuclei in the course of development. 



Functionally the nucleus is regarded as a center for chemical activities 

 necessary for the life of the cell. It is believed to produce substances 

 which pass out into the cytoplasm, where they may be further elaborated. 

 Evidences of nuclear extrusions into the cytoplasm have been frequently 

 recorded. But the interactions between nucleus and cytoplasm, of such 

 nature that they cannot be observed under the microscope, are presum- 

 ably of far greater biological importance. 



CENTROSOME. 



The centrosome is typically a minute granule in the center of a small 

 sphere of differentiated protoplasm. Often the term is applied to this 

 entire structure, but it refers particularly to the central granule; the 

 enveloping sphere is known as the attraction sphere, and it is composed 

 of archoplasm^ When a cell is about to divide, delicate fibrils, either re- 

 arranged from the protoplasmic reticulum or formed anew, radiate from 

 the archoplasm toward the periphery of the cell. The central granule 

 becomes subdivided into two, which then move apart. In resting cells, 



