THE CELL AND THE TISSUES. 13 



interstitial, seemingly less active substance the hyaloplasm. The 

 active contractility which has been generally credited to the spon- 

 gioplasm has been questioned (Schaefer), since the characteristic 

 amoeboid movements of living cells are by some attributed to the 

 changes taking place within the hyaloplasm. 



The arrangement of these constituents of the protoplasm is vari- 

 able. When they exist closely and uniformly intermingled, the 

 customary finely granular appearance of the cell-contents is produced; 

 not infrequently, however, the spongioplasm is disposed as a more 

 or less well-defined reticulum. In living cells this reticulation is 

 transient, and, to a certain degree, acci- 

 dental, since it often depends upon an 

 unequal distribution of the hyaloplasm 

 induced by the presence of vacuoles or 

 of particles of foreign substance, as se- 

 cretion within glandular epithelium. 



Chemically, protoplasm consists of 

 various albuminous substances in com- 

 bination with a special nitrogenous pro- 

 teid, plastin, together with water and 



SaltS. It is probable that in the albu- Structure of the cell : a, spongio- 



minous substances alone the property of '^^^7^ 



Contractility resides; the plastin, On the wall ; c, chromatin filaments, between 



other hand, offers great resistance to *-JS? *"*" 



those reagents, as acids, gastric juices, 



or trypsin, which dissolve the albuminates. The amount of plastin 

 present within the fibrils forming the intercellular reticulum is not 

 constant, but subject to considerable variation. In addition to the 

 hyaloplasm, the meshes of the spongioplasm frequently contain 

 particles of foreign substances ; the latter may be fatty matters, 

 pigment granules, particles of secretion elaborated within the cell 

 itself, or extraneous material. 



The nucleus is usually limited by a distinct wall, the nuclear 

 membrane, and is traversed by a variably elaborate framework of 

 nuclear fibrils, between which lies an interfibrillar, probably semi- 

 fluid, substance, the nuclear matrix. The nuclear membrane is 

 composed of amphipyrenin. The nuclear fibrils consist of particles 

 of a deeply staining proteid substance, chromatin, supported by 

 delicate achromatic threads of linin, along which the chromatin is 

 arranged as irregular beads or thickenings. 



Suspended within the nuclear net-work, lying often in close rela- 

 tion with the fibrils, one or more minute spherical bodies may be 

 seen ; these are the nucleoli, regarding whose true significance, at 

 present, little is definitely established. The nucleolus is highly 



