MITOSIS 49 



granules of chromatin scattered over a new network of linin, all formed 

 out of the chromatin. A new nuclear membrane appears about each 

 group and thus two nuclei take form; this membrane is also formed 

 from the chromatin, though the cytoplasm may assist in its formation. 

 The spindle fibers gradually disappear, as do the astral rays. A constric- 

 tion appears in the cytoplasm of the cell in the same plane as the equa- 

 torial plate, frequently as early as the stage in which the two groups of 

 chromosomes begin to separate. This constriction grows deeper and 

 deeper while the two new nuclei, or daughter nuclei, are being formed. 

 Finally, as the spindle fibers disappear, the constriction cuts clear through 

 the cell, which thus forms two new cells, each with a nucleus, containing 

 one-half of the original chromatin material and each half the size of the 

 parent cell. 



The whole process of cell division which has been outlined is divided 

 into four phases. All of the steps from the beginning to the time when 

 the chromosomes line up in the equator of the spindle are termed collec- 

 tively the prophase. The steps involving the splitting of the chromo- 

 somes into pairs in the equatorial plane are termed the metaphase. 

 The process of division may be arrested for a time at this point. The 

 period of migration of the daughter chromosomes from the equator of 

 the spindle to the poles is termed the anaphase, and the series of steps 

 involved in the division of the cytoplasm of the cell and the construction 

 of two separate nuclei is termed the telophase. 



These steps do not always occur in the same order and any general 

 outline such as has been given will have to be modified in a great variety 

 of ways to suit different cases. There may be two centrioles in the 

 resting cell before mitosis begins. Variations occur in the time of the 

 splitting of the chromatin thread and its division into chromosomes. 

 In some cases the spireme is single and breaks transversely into chromo- 

 somes during the prophase. These line up on the equator of the spindle 

 and split longitudinally in the metaphase. The two of a pair so formed 

 are similar to the parent chromosome, and it is assumed that each pair of 

 chromosomes formed from the double thread is derived from a parent 

 chromosome, to which they are similar. Sometimes the linin seems to 

 disappear; at other times it seems to assist in the production of the 

 spindle ; and in still other cases it seems to contribute to the formation of 

 the chromosomes. In some cases, especially when the chromosomes form 

 a ring around the spindle, a distinction may be drawn between the chro- 

 mosomal or traction fibers which connect the chromosomes with the 

 poles of the spindle, and which are also called mantle fibers because they 

 are on the outside of the spindle, and the continuous fibers that run 

 from one pole to the other. This mode of cell division has been called 

 karyokinesis, meaning nuclear movement, or, more commonly, mitosis, 

 from the Greek word for thread, referring to the chromatin thread. 



