4 NORMAL HISTOLOGY. 



corresponds in shape with the form of the cell, being oval or rod-like in 

 elongated columnar or fusiform cells, and compressed or flattened in plate- 

 like elements, its outline is sometimes very irregular, as conspicuously seen 

 in the case of the colorless cells (leucocytes) of the blood. At times the 

 nucleus is capable of changing its form or even position independent of 

 the surrounding cytoplasm. Except during certain phases of division, when 

 the usual demarcation temporarily disappears, the nucleus is sharply defined 

 from the cytoplasm by a distinct envelope, the nuclear membrane. The latter 

 encloses the substance of the nucleus, the karyoplasm, which structurally 

 resembles the cytoplasm in being composed of two parts an irregular 

 reticulum of nuclear fibrils and an intervening semifluid nuclear matrix. 



The nuclear fibrils, when examined under high magnification after 

 appropriate treatment with suitable stains, such as hematoxylin, safranin 

 and other basic dyes, are shown to consist of minute irregular masses of a 

 deeply colored substance, appropriately called chromatin, in recognition of 

 its great affinity for certain stains. The chromatin particles are supported 

 upon or within delicate inconspicuous and almost colorless threads of linin. 

 The latter forms the basis of the supporting framework of the nuclear fibrils, 

 in which the chromatin is so conspicuous by reason of its capacity for stain- 

 ing. The individual masses of chromatin vary greatly in form, often being 

 irregular, and at other times thread-like or beaded. Not infrequently the 

 chromatin presents spherical aggregations which appear as deeply stained 

 nodules attached to the nuclear fibrils. These constitute the false nucleoli, 

 or karyosomes, as distinguished from the true nucleolus, which is usually 

 present within the karyoplasm. Chemically, chromatin, the most important 

 part oi the nucleus, contains nuclein, a compound rich in phosphorus. 



The nuclear matrix, the fluid or semifluid substance which occupies 

 the spaces between the nuclear fibrils, possesses an exceedingly weak affinity 

 for the staining reagents employed to color the chromatin. It usually appears, 

 therefore, clear and untinted, and contains a substance described as paralinin. 

 The nucleolus, or plasmosome, ordinarily appears as a small spherical 

 body, sometimes multiple, lying among, but unattached to, the nuclear 

 fibres. In stained tissues its color varies, sometimes resembling that of the 

 chromatin, although less intense, but usually presenting a different tint, 

 since it responds readily to dyes which, like eosin or acid fuchsin, particu- 

 larly affect the linin and cytoplasm. Concerning the nature, purpose and 

 function of the nucleolus much uncertainty exists. According to some 



authorities these bodies are to be regarded 

 ^^^^ as storehouses of substances which are used 

 /T Rb in forming the chromatin segments during 

 cell-division, while other cytologists attribute 

 to the nucleolus a passive role, even holding 

 it to be a by-product, which, at least in 

 some cases, is cast out from the nucleus 

 into the cytoplasm, where it may disappear. 



P.O. t.-A. intestinal epitheimm with The nucleolus is credited with containing a 

 paired centrosomes. c. x&o. (Zimmer- peculiar substance known as paranuclein. 



mann.) B, epithelium from pancreas TU r~. T JJ'..' ,.i_ 



with mitochondria. X725- (Saguchi.) 1 he centrosome. In addition to the 



parts already described, many animal cells 



contain a minute body, the centrosome, which probably plays an important 

 role during division and, in a lesser degree, during other phases of cellular 

 activity. Ordinarily the centrosome escapes attention because, on account 

 of its minute size and variable staining affinity, it is with difficulty distin- 



