THE ANIMAL CELL. 9 
substance). There is also a coarser network, more readily stainable, consisting of 
chromatin, granular portions of which may also be embedded in the linin. Instead 
of forming a coarse network the chromatin may be arranged in the form of a con- 
voluted cord, or as a number of separate filaments, and in certain cases it constitutes 
a series of loops from which secondary branches are projected, the apices of the 
loops being grouped together at one pole of the nucleus round a clear area known 
as the “polar field.” 
The nuclear membrane consists of both chromatin and linin. 
Nucleoli are of two kinds, true and false. A true nucleolus is a small, 
refractile particle,-of spherical outline, embedded in the reticulum. It stains 
deeply, and is said to consist of a special modification of the karyoplasm which is 
called pyrenin. False nucleoli are simply the nodes of the chromatin reticulum. 
The nucleus is capable of motion; it has been seen to alter its shape in 
the living cell, and it undoubtedly plays an active part in the process of cell 
reproduction. 
In addition to the nucleus many cells contain one or more small rounded bodies 
called centrosomes. Possibly they are only condensed portions of the cyto- 
reticulum. They lie within a clear space which is known as an attraction 
sphere, from which numerous fine lines radiate. 
Centrosomes become very evident when reproduction commences, but are not 
so distinct at other times. 
The attraction sphere also becomes more evident when cell-division commences, 
and the contained centrosome as well as the radii which project from it appear to 
play important parts in the reproductive process. 
Reproduction of Cells.—Cell division or reproduction may take place either— 
1. By direct division—amitosis ; 
2. By indirect division—amitosis or karyokinesis. 
In the amitotic or direct form of division the nucleus, and then the cell body, 
are equatorially constricted, the constrictions deepen until both are completely 
divided, and so two daughter cells are produced. Apparently the attraction sphere 
and centrosome play some part in this process, but whether their influence is 
initiative or directive is unknown. 
Fic. 3.—CELL DIVISION. 
Successive stages of mitosis or karyokinesis (diagrammatic, modified from Henneguy). A, B, C, D, and E 
illustrate the phenomena of the prophase ; F those of the metaphase ; G and H those of the anaphase ; 
J, K, and L those of the telophase. 
Mitosis, the process of indirect division, is by far the most common mode of 
cell-division. It is a complex process, and the phenomena observable during its 
progress are classified into four groups: (1) the prophase, (2) the metaphase, (3) the 
anaphase, and (4) the telophase. 
