NUCLEUS. 5 



deeply staining bodies. The ground substance and network are closely 

 related to the corresponding structures in the protoplasm. In fact, at the 

 time of cell division, when the nuclear membrane disappears, the network 

 and ground substance of nucleus and protoplasm are respectively con- 

 tinuous with one another. The ground substance and a delicate, fibrillar 

 portion of the network do not stain readily; therefore they are called 

 achromatic substances. The fine achromatic fibrils of the network are 

 further designated as linin fibrils. Since the linin fibrils cannot be isolated 

 for chemical analysis, their composition is unknown. There are two- 

 deeply staining or chromatic substances found in the nucleus. One of 

 these, chromatin, is its most essential and characteristic element. Chemi- 

 cally it is a nucleo-proteid, but undoubtedly it exists in several varieties. 

 A portion which responds to acid dyes is called oxychromatin, in distinc- 

 tion from the ordinary form which takes the basic stains. Chromatin is 

 distributed as irregular granules or coarse strands along the linin fibrils, 

 thus tending to form a network (Fig. i, and Fig. 18, p. 16). Often a 

 nucleus presents from one to several large clumps of chroma tin, known 

 as chromatin "knots." These are to be distinguished from the round 

 masses of pyrenin, called nucleoli, which are found between the meshes 

 of the nuclear network. All nuclei contain chromatin, but many are 

 without nucleoli. The latter are present with great regularity in certain 

 kinds of cells. Usually only one is found in a nucleus, although several 

 may occur (Fig. 18). They differ from chromatin chemically, as is 

 evident from differences in staining, and also functionally, as is seen during 

 cell division. Pyrenin, of which the nucleolus is said to be composed, is, 

 however, a cytological rather than a chemical term. 



The nuclear membrane is usually described as formed of amphipy- 

 renin, a term of questionable value. The membrane may consist of a dis- 

 tinct chemical substance as the name suggests, or it may be rather a conden- 

 sation of the nuclear reticulum, in which the linin fibrils terminate. A nuclear 

 membrane may be simulated by a thin superficial layer of chromatin. 



Every cell contains a nucleus consisting, as has been shown, of nuclear 

 membrane, ground substance, a network of linin fibrils and of chromatin, 

 with perhaps a nucleolus. Xon-nucleated bodies like the mammalian 

 red blood corpuscles, and the dead outer cells of the skin have lost their 

 nuclei in the course of development. Occasionally a single cell contains 

 two nuclei, as is frequent in the liver, or even several nuclei, as in certain 

 bone cells. 



CENTROSOME. 



The centrosome is a minute body consisting of a homogeneous or 

 sometimes reticular mass, the centroplasm, which contains a much smaller 



