1598 A MANUAL OF ANATOMY 



In most cells, usually close to the nuclear membrane, but external 

 to it, there is a small spherical area of cytoplasm, from which lines 

 radiate outwards into the cell-protoplasm. This area is called the 

 centrosome or attraction-sphere, and the protoplasm around the 

 area is known as the archoplasm. At the centre of the centrosome 

 there are usually two small nodules of protein matter, called the 

 central or aitraction-farticles, from which lines radiate outwards into 

 the archoplasm and cjrtoplasm. The centrosome thus constitutes the 

 aster, and it plays an important part in nuclear division by mitosis. 



The cell-nembrane, when present, is a condensation of the peri- 

 pheral c5rtoplasm. In many cells, however, it is absent. 



The Nucleus. — 'The nucleus is usually situated eccentrically in 

 the cytoplasm. Its protoplasm is called karyoplasm, and the 

 nuclear elements are as follows : 



1. Nuclear membrane. 3. Karyoplasm. 



2. Nuclear reticulum. 4. Nucleoli. 



The nuclear membrane is a well-defined envelope which surrounds 

 the nuclear contents and separates them from the cytoplasm. It 

 consists of the elements of the nuclear reticulum — namely, nuclein 

 containing chromatin, and linin. 



The nuclear reticulum, which corresponds to the spongioplasm of 

 the cell-protoplasm, consists of nuclein, containing a stainable 

 material called chromatin, arranged in granules. These granules 

 are connected by threads of linin. 



The karyoplasm, which corresponds to the hyaloplasm of the 

 cell -protoplasm, occupies the meshes of the nuclear reticulum, and 

 contains granules, known as karyosomes. 



The nucleolus (sometimes absent) may be one or more in number. 

 There are two kinds of nucleoli — ^true and false. The true nucleoli 

 lie in the nuclear reticulum, or, it may be, in the karyoplasm. The 

 false nucleoli are nodes which are connected with the filaments of 

 the nuclear reticulum, where they intersect. 



Cell-Division. 



There are two kinds of cell-division — namely, karyokinetic or 

 mitotic, which is indirect division, and akinetic or amitotic, which 

 is direct division. 



Karyokinesis or Mitosis. — ^This kind of cell-division is of a very 

 complicated nature, and the changes involved affect both the 

 nucleus and the centrosome. It is convenient to consider it under 

 four phases — namely, (i) the anaphase, (2) the metaphase, {3) the 

 kataphase, and (4) the telophase. 



Anaphase. — ^The anaphase constitutes the preparatory stage, and 

 it includes three phenomena, all of which lead ' up ' to the meta- 

 phase, as follows: 



I. Formation of spireme. 2. Formation of chromosomes. 



3. Formation of spindle. 



