MORPHOLOGY OF THE CELL 37 



interpreted with caution, however, for if an actual fiber is stretched 

 through the emulsion the alveoh near it must be arranged in rows. 

 Experiments with artificial emulsions indicate that this rearrangement 

 does occur. As pointed out below, the nucleus also shows undoubted 

 fibers. These observations led, in the early days of cell studies, to what 

 were known as "filar" and "reticular" theories of protoplasmic struc- 

 ture. These threads are probably of importance, although their function 

 is not well understood. 



Killed, hardened, and stained protoplasm usually reveals multitudes 

 of granules. These are coagulation products of the interalveolar material 

 in most cases, the cut ends of fibers viewed on end in other cases, and 

 sometimes cell inclusions. 



The bulk of protoplasm is, as has been seen, an emulsion. The spheres 

 and the liquid in which they float constitute what the physical chemist 

 calls "phases" of a "system." Certain phenomena characteristic of 

 such systems, particularly a variety of surface phenomena, are thus 

 phenomena of protoplasm. The surface phenomena are intensified by 

 the colloidal nature of the substances which are present even when the 

 protoplasm does not appear to be an emulsion. Much of the activity 

 of protoplasm is due to these surface phenomena. 



Finally, it should be made clear that protoplasm is in a liquid or semi- 

 liquid state. Solid particles, except as cell inclusions, are unknown in it. 

 It is plastic. Substances may dissolve and diffuse in it. These physical 

 properties are important in the consideration of the physiology of proto- 

 plasm, to be undertaken in the next chapter. 



Structure of the Nucleus. — The nucleus differs, in its minute structure, 

 from the rest of the cell. It is enclosed within a thin membrane, called 

 the nuclear membrane (Fig. 20, nm), which is differentiated out of the 

 general protoplasm. Its composition appears to be not always the same. 

 Within this membrane the bulk of the nucleus consists of a liquid, the 

 nuclear sap (ns), probably with various substances in solution. Stretched 

 through this fluid is, in the most common cases, a network of fine, anasto- 

 mosing filaments of a substance named linin (/). This substance stains 

 only slightly, and is but dimly seen in most prepared cells. However, 

 its position is made evident by another substance which is attached to, or 

 imbedded in, the linin. This substance is called chromatin {ch), from its 

 high affinity for dyes. In stained cells it is very conspicuous, and gives 

 the nucleus its color in such preparations. The chromatin appears in 

 the form of granules or minute masses called chromomeres; sometimes 

 these masses are conspicuously larger at certain places than at others, 

 and are known as net-knots. 



The network of linin and chromatin is most commonly distributed 

 throughout the nucleus without evident arrangement. Sometimes, 

 however, it has a characteristic configuration. It may consist of a ribbon- 



