THE CFXL 49 



It may be complete, or fenestrated allowing free communication be- 

 tween the cytoplasm and the nuclear contents. 



(b) The intranuclear network, or nucleoreticidum, consists of a 

 chromatic element {nuclein or chromatin) and of an achromatic element 

 (linin). The linin constitutes the groundwork of the reticulum along 

 which the chromatin granules are distributed. At nodal points 

 of the network there are often considerable accumulations of chroma- 

 tin. These nodal points, at first thought to be nucleoh, are now 

 known as false nucleoli, or karyosomes. Instead of a distinct network 

 there may be disconnected threads or simply granules of chromatin. 

 Chromatin is the most characteristic of the chemical constituents 

 of the nucleus, the only one which contains phosphoric acid, and also, 

 apparently, the only nuclear substance which is always transmitted 

 from parent to daughter cell in cell-di\dsion. Fine granules have been 

 described as occurring in the linin, differentiated from chromatin by 

 the fact that they are most susceptible to acid dyes, while chromatin 

 takes basic dyes, 



(c) The nucleolus or plasmosome {paranuclein, pyrenin) is a small 

 spherical body within the nucleus. Not infrequently there are sev- 

 eral nucleoli. Similar cells vary as to the number of nucleoli they 

 contain. The same cell may vary as to the number of its nucleoli 

 under varying functional conditions. The nucleolus regularly dis- 

 appears during mitosis, and during functional activity in some 

 gland cells. It stains intensely with basic dyes. Its function is 

 unknown. 



{d) Karyoplasm {nucleoplasm, nuclear fluid , nuclear sap). This is 

 the fluid or semi-fluid material which fills in the meshes of the nucleo- 

 reticulum. 



While the nucleus is a perfectly distinct structure capable in 

 some animal and in some vegetable cells of moving about more or 

 less actively in the cytoplasm, and is usually separated by a membrane 

 from the rest of the cell, a marked similarity exists between the 

 structure of nucleoplasm and cytoplasm. This similarity is empha- 

 sized by the absence in some resting cells of any nuclear membrane, 

 by the apparent direct continuity in some cases of nucleoreticulum 

 and cytoreticulum, and by the continuity of karyoplasm and cyto- 

 plasm in all cells during cell-division. 



4. The centrosome (Fig. 5) is a small spheroidal body found 

 sometimes in the nucleus, or more commonly in the cytoplasm near 

 the nucleus. In actively dividing cells the centrosome is frequently 



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