THE NUCLEUS. 63 



In treating the nuclei with certain stains, the nuclear reticulum will 

 be seen to consist of two constituents, a substance appearing in 

 the form of variously shaped, minute granules, which stains deeply, 

 and is, therefore, known as chromatin. This is imbedded in and 

 deposited on a less stainable network, the limn. The meshes of this 

 network are occupied by a transparent, semifluid substance, which 

 does not stain easily, and is known as the achromatic portion of the 

 nucleus. It is also known as paralinin, nuclear sap, karyolymph, or 

 nucleoplasm. Chemically, chromatin belongs to those albuminous 

 substances known as nucleins. 



In well-stained nuclei of considerable size the chromatin gran- 

 ules are seen closely placed in a continuous row throughout the net- 

 work of linin, which penetrates the nuclei in all directions. In 

 every resting nucleus one or more small round bodies are found 

 imbedded in the nucleoplasm. These are known as true nucleoli, 

 and do not stain quite so deeply as the chromatin. The fact that 

 certain reagents dissolve the chromatin, but not the true nucleoli, 

 proves that the substance of which the latter are composed is not 

 identical with chromatin, and is, therefore, known as paranuclein 

 (F. Schwartz). 



In many cases we find in the linin, granules of a substance 

 known as lanthanin, which displays a marked affinity for the so- 

 called acid anilin stains, in contradistinction to chromatin, which 

 stains principally with the basic anilin colors. These are known as 

 oxychromatin granules in contradistinction to the basichromatin 

 granules of the chromatin (M. Heidenhain, 94). 



The true nucleoli should not be confused with the slight swell- 

 ings of the chromatin network found at the junction of the threads, 

 and known as net-knots, or karyosomes. 



Surrounding the resting nucleus is usually a nuclear membrane 

 (amphipyrenin) resembling in many respects chromatin. As a rule, 

 it does not form a continuous layer, but is perforated, having open- 

 ings that contain nuclear fluid. We have, then, both substances, 

 chromatin and nucleoplasm, as elements of the nuclear membrane. 

 Besides this, the nuclear membrane receives an outer layer, differ- 

 entiated from the protoplasm. Later investigations have shown 

 that even during a period of rest the relationship of the nucleus to 

 the protoplasm of the cell is much more intimate than was hereto- 

 fore believed (vid. Reinke, 94). 



A resting nucleus i.e., one not in process of division usually 

 consists, therefore, of a sharply defined membrane (amphipyrenin), 

 which has in its interior a chromatic (nuclein) and an achromatic 

 (linin) network, a nuclear fluid (paralinin), and nucleoli (paranuclein). 



The chromatin of the nucleus is not always in the form of a net- 

 work. In some cases as, for instance, in the premature ova of 

 certain animals (O. Hertwig, 93. II) and in spermatozoa it is col- 

 lected in compact bodies. In the ova it may often be mistaken for 

 a true nucleolus (germinal spot). In this case, however, it consists 

 of nuclein, and not of paranuclein. 



