CELL DIVISION 71 



indirect cell division. The word karyokinesis is derived from two Greek 

 words which mean nut or nucleus and change or movement. The word 

 mitosis is from a Greek word meaning a thread. As will be seen in the 

 description of indirect cell division, the word karyokinesis is more de- 

 scriptive of the process than is the term mitosis, since the latter is de- 

 scriptive only of a very brief stage of the process which in some cells may 

 not occur at all. Nevertheless, the name mitosis, introduced by Flem- 

 ming in 1879, is now more commonly used than karyokinesis, proposed 

 by Schleicher in 1878. 



GENERALIZED ACCOUNT OF MITOSIS 



The scheme of mitosis here outlined deals with the process in animal 

 cells in which a centrosome is present, and is applicable with certain 

 modifications and reservations to most cases of indirect cell division in 

 animal cells. The process involves a series of changes in the nucleus, the 

 cytoplasm and the centrosome which are actually parallel or sj-nchro- 

 nous, but the description of the process cannot follow the precise chrono- 

 logical order and be made clear. For convenience in description and 

 discussion the process may be divided into four stages or general phases 

 which have no sharply defined limits. These stages are (1) the prophases, 

 that is, the phases from the beginning which lead up to, but do not include, 

 the splitting of the chromosomes, (2) the metaphase, involving the actual 

 splitting of the chromosomes, the most important stage of all, (3) the 

 anaphases, the phases in which the chromosomes are distributed, and 

 (4) the telophases, involving the division of the body of the cell, the forma- 

 tion of daughter cells, and the reconstruction of the daughter nuclei. 



The Prophases. — Prior to the beginning of mitosis the chromatin of 

 the nucleus is arranged in the form of a network or as scattered granules 

 supported by the linin network. This arrangement of chromatin is 

 shown in Figs. 20 and 35. Occasionally where large fragments of the 

 chromatin network cross each other rounded masses may be noted. 

 These masses are called net-knots. As indicated in Chapter II, they 

 are entirely distinct from nucleoli, as may be determined by the use of 

 proper staining methods. On the approach of division the chromatin 

 undergoes changes in form, and as indicated by its greater affinity for 

 dyes it apparently also undergoes changes in its chemical constitution. 

 It becomes condensed into a very fine thread which in some species seems 

 to be continuous and in others discontinuous. This thread which is at 

 first very fine and closely coiled is usually called a close skein or close 

 spireme (Fig. 36) because the threads are near each other, giving a 

 closely tangled appearance. Even at this early stage of division the 

 chromatin stains more intensely than did the chromatin granules of the 

 resting nucleus. As the process continues the thread thickens and 



