According to recent research, most living cells also contain a minute particle (usually lying near the nucleus) which is termed the attraction particle or centrosome because it attracts the protoplasmic granules. The spherical fibrillar rows of granules surrounding the particles are known as the attraction-sphere or centrosphere, and it is usually double and connected by a spindle-shaped system of delicate fibrils called the acromatic spindle. They are mostly easily seen in young cells about to undergo division, which is believed to commence there.
The process of cell reproduction happens either through indirect or direct division. Indirect division (known as karyokinesis or karyomitosis) has been witnessed in all types of cells, and it will likely be proven that direct division is only an imperfect or abnormal karyokinesis.
Indirect cell division involves a number of complex changes to the nucleus which result in its subdivision and then the cleaving of the cell protoplasm. These changes can be grouped under the four headings of prophase, metaphase, anaphase and telophase.
1. Prophase:
The nuclear network of chromatin filaments becomes a skein or spirem, while the nuclear membrane and nucleolus disappear (Skein or Spirem, Close). The convoluted skein of chromatin divides into a number of V-shaped loops known as chromosomes (Skein or Spirem, Open). Preceding or at the same time as this, the attraction sphere splits into two parts connected by the achromatic spindle. The two centrosomes move away from each other, one toward each end of the nucleus, and the fibrils of the achromatic spindle are lengthened. The centrosomes are now situated at opposite poles of the spindle, and each is surrounded by a centrosphere from which radiate the fibrils into the investing protoplasm. A line encircling the spindle midway between the ends is called the equator, and around this the V-shaped chromosomes form a star, known as a motherstar or monaster (Motherstar, Monaster).
2. Metaphase:
Each chromosome now undergoes cleavage into two equal V-shaped halves, or daughter chromosomes. The daughter chromosomes travel in opposite directions down the fibrils of the achromatic spindle toward the centrosomes, around which they group themselves in star like figures. These two groups are then called the diaster.
3. Anaphase:
The V-shaped daughter chromosomes now form their individual skein or spirems and eventually form the network of chromatin and the nuclear membrane and nucleolus. The protoplasm begins to appear constricted around the equator of the achromatic spindle and double rows of granules are sometimes seen. The constriction continues and the original cell is gradually divided.
4. Telophase:
The cell is completely divided into two separate cells with all parts of the cells in the correct place for resting cells.
In a prickle cell, the subdivision is incomplete, leaving achromatic spindle threads bridging across the intercellular space.
Direct division is brought about by either segmentation (fission) or gemmation. In the former, the nucleus becomes constricted in the center in the form of an hourglass, then divides into two, which leads the protoplasm to do so as well. These daughter cells are smaller than the original, but grow quickly and can divide in the same manner for fast multiplication. In gemmation, a portion of the nucleus buds off or separates and becomes its own organism.
The cell wall, an occasional part of a cell, is a firmer external layer of protoplasm, sometimes thickened by chemical deposits, that forms a flexible transparent membrane.
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