STRUCTURE OF THE CELL. 9 



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 /^■;^^>^. The latter, therefore, forms the supporting net-work of the nuclear 

 fibrils in which the chromatin is so prominent by virtue of its capacity for staining. 

 The forms of the individual masses of chromatin vary greatly, often being irregular, 

 at other times thread-like or beaded in appearance. Not infrequently the chromatin 

 presents spherical aggregations which appear as deeply stained nodules attached to 

 the nuclear fibres ; these constitute the false nucleoli, or karyosomes, as distinguished 

 from the true nucleolus which is frequently present within the karyoplasm. Chemi- 

 cally, chromatin, the most essential part of the nucleus, contains miclehi, a com- 

 pound rich in phosphorus. 



The matrix, or nuclear juice, which occupies the interstices of the net-work, 

 possesses an exceedingly weak affinity for the staining reagents employed to color 

 the chromatin, and usually appears clear and untinted. It is probably closely related 

 to the acJu'omatin and contains a substance described 2s paralinin. 



The micleolus, or plasmosome, ordinarily appears as a small spherical body — 

 sometimes multiple — lying among, but unattached to, the nuclear fibres ; its color in 

 stained tissues varies, sometimes resembling that of the chromatin, although less 

 deeply stained, but usually presenting a distinct difference of tint, since it responds 

 readily to dyes which, like eosin or acid fuchsin, particularly affect the linin and 

 cytoplasm. Concerning the exact nature, purpose, and function of the nucleolus 

 much uncertainty still exists ; according to certain authorities, these bodies are to be 

 regarded as storehouses of substances which are used in the formation of the chro- 

 matin segments during division, while other cytologists attribute to the nucleolus a 

 passive role, even regarding it as by-product which, at least in some cases, is cast out 

 from the nucleus into the cytoplasm, where it degenerates and disappears. Since 

 trustworthy observations may be cited in support of both of these conflicting 

 views, definite conclusions regarding the exact nature of this constituent of the 

 nucleus must be deferred. The nucleolus is credited with containing a peculiar 

 substance known as pyrenin. The term amphipyrenin, as applied to the substance 

 of the nuclear membrane, is of doubtful value. 



The Centrosome. — In addition to the parts already described, which are con- 

 spicuous and readily seen, the more recent investigations into the structure of cells 

 show the presence of a minute body, the cen- 

 trosome, which plays an important role in Fig. 7. 



elements engaged in active change, as con- ^^m^ ^ _«_,. 



spicuously durmg division and, in a lesser j^^^^^^Hj ^■-^— ". ; /m '■'■ 7 

 degree, during other phases of cellular activity. ^^^^^^^■'^ W ^c^ajjw '^^ 



Ordinarily the centrosome escapes attention IBHIIHi i P PP -^^i' "^r i^ 



because, on account of its minute size and varia- ■*' z^' V *"' 7" 

 ble staining affinity, it is with difficulty distin- ^■^' '*- ." 



guished from the surrounding particles. Its n --^^ 



usual position is within the cytoplasm, but the C ii^~^^ ^S^ 



exact location of the centrosome seems to de- •■  •.— c lfi^V>-.-i ^^W~^ 



pend upon the focus of greatest motor activity, .^f:'^ 1 ; 



since, as shown by Zimmermann, this little 

 body, or bodies, being often double, is always 

 found in that part of the cell which is the seat 

 of greatest change ; thus, in a dividing ele- 

 ment, the centrosome lies immediately related centrosomes (., c) i„ human epithelium; 



to the actively changing nucleus, while within a, B, cells from gastnc glands; C, from duo- 

 •!• . J vt- 1* -r • 1 X -„ 4.1, denal glands; /?, from tongue ;/, leucocyte with 



Ciliated epithelium It is removed from the nu- centrolome X 625. i^K. IV. zimmermann.) 



cleus and is found closely associated with the 



contractile filaments which probably produce the movements of the hair-like ap- 

 pendages. In recognition of the intimate relations between this minute body and the 

 active motor changes affecting the morphological constituents of the cell, the cen- 

 trosome may be regarded physiologically as its dynamic centre ; the name ki7io- 

 ce7itrum has been suggested by Zimmerman as best expressing this probable 

 function of the centrosome. This little body is frequently surrounded by a clear 



