THE CELL 17 



this process, a single small j^raiuilar body, the centriole, is surroiiiuled 

 by a clear area of cyto])lasni, the centros])here. After treatment 

 with ])n)tein-])recii)itatins ao:ents, numerons fine ])rotoplasmic rays 

 may be made out, extending in all directions into the cytoi)lasm 

 from the centros])here. The so-called astrosphere thus formed is 

 not connnonly visil)le in living material nor in material otherwise 

 fixed. The function of this ai)paratus is not clearly understood, 

 l)ut ajjpears to be intimately connected with mitosis. 



In addition to the bodies already mentioned, there are other less 

 common })articles distributed in the cytoplasm. In nerve cells and 

 striated muscle fibers, abundant and distinct fibrils may occupy 

 much of the cytosome. The cells in the early dixisions of the frog 

 embryo contain yolk granules, and glycogen can be demonstrated 

 as a stored substance in the cytoplasm of liver cells. Under favor- 

 able conditions, droplets of fat form in certain connective-tissue 

 cells, (iranules of pre-enzymatic nature are recognizable in gland 

 cells, and pigment granules occur in the cytoplasm of certain epithe- 

 lial and connective-tissue cells. 



A limiting membrane is not clearly evident in all cells. Some 

 investigators believe it to be merely a condensation of the peripheral 

 cytoplasm which probably contains lipins. By using microdissec- 

 tion apparatus it has been possible to indicate that this membrane 

 possesses some degree of elasticity. It has been demonstrated to 

 be semipermeable and appears to regulate the passage of oxygen 

 and nutriti\e materials into the cell cytoplasm as well as the passage 

 of carbon dioxide and nitrogenous wastes out of the cell. The 

 exposed surface of some cells may have a thick, tough external 

 cuticle or other structural modifications. Certain groupings of cells 

 are not completely separated by indi\'idual cell membranes and 

 form a syncytium as in the case of cardiac and skeletal muscle. 



Nucleus. — In general, the nucleus of the cell is a globular or ovoid 

 body and is separated from the cytoplasm by a definite membrane. 

 \Yithin the membrane, the nucleus is composed of a homogeneous 

 watery fluid, containing special proteins, the nucleoproteins or 

 chromatin in colloidal solution. Fixation and use of basic stains 

 bring out a granular network of chromophilic substance that 

 increases in density as organization of the nucleus for division 

 advances, until the chromosomes, which are mainly solidly staining 

 bodies composed of nucleic acid, stand out clearly. Cytogenetics 

 has demonstrated that it is the chromatin material of the nucleus 

 which carries the hereditary factors. Nucleus and cytoplasm diflfer 



2 



