8 



HISTOLOGY 



ance to the nucleus. Sometimes certain portions of the chromatin are 

 in the form of bodies that greatly resemble nucleoli (Fig. 6). Although 

 such chromatin bodies and nucleoli both stain deeply, the shade and 

 quality of stain show them to be different. The nucleoli or plasmosomes 



ch nu 



FIG. 6. Motor nerve cell from electric lobe of brain of Tetronarce. imp.c., implantation cone 

 and beginning of neurite; den., dendrites; ch.nu., chromatin knot. X 1500. 



stain less intensely than the chromatin masses. These chromatin masses 

 are called karyosomes or chromatin knots (see Fig. 6). Being composed 

 of a number of individual chromatin grains, they are rough and irregular, 

 while the plasmosomes are smooth in outline. The nucleus sometimes 

 contains another structure, the centrosome, which, as it is usually found 



lying outside the nucleus in the cytoplasm, 

 will be described in connection with the 

 cytoplasm. 



The cytoplasm may be looked upon as 

 the least differentiated part of the cell of 

 higher organisms. The general study of 

 protoplasm has mostly, if not always, been 

 based upon the cytoplasm. Hence what 

 was said in Chapter I of the texture of 

 protoplasm will apply to cytoplasm. It 

 usually presents an alveolar appearance. 

 The alveoli may be so small that the cyto- 

 plasm appears not to be alveolar. Numer- 

 ous minute granules always form part of the cytoplasm. They are the 

 micro somes (see Fig. i). Fibrillae are frequently present. They are to be 

 considered as differentiated cytoplasmic structures which have to do 

 with certain activities of the cell. They seem to be closely associated 

 with certain forms of cytoplasmic activity, with secretion and excretion, 



sec. 



FIG. 7. Pancreas cell from salaman- 

 der, sec., secretion substance; _fi., 

 secretion fibrils. After MATHEWS. 



