SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 145 



many nuclei are distinct from each other from the first, whereas in other 

 cases they may be arranged end-to-end in a more or less continuous 

 thread, or spireme, which later segments transversely into independent 

 chromosomes. The slender threads (chromosomes) now split longitu- 

 dinally throughout their entire length. A progressive shortening and 

 thickening of the split threads ensues, so that the nucleus is eventually 

 seen to have a certain number of chromosomes which have become 

 double through a longitudinal cleavage. 



While the above changes are occurring, fine fibrils are differentiated 

 in the cytoplasm near the nucleus and become arranged in two opposed 

 groups. The nuclear membrane now disappears and the fibers extend 

 into the nuclear region, where some of them (the "mantle fibers") 

 attach themselves to the double chromosomes, while others (the "con- 

 necting fibers") pass through from one pole to the other. The double 

 chromosomes quickly become arranged in a single plane at the equator 

 of the cell, the fibers meanwhile forming the achromatic figure, or spindle. 

 This stage is known as the metaphase; all the steps leading up to it, be- 

 ginning with the initial changes in the resting reticulum, constitute the 

 prophase. 



The daughter chromosomes (the halves of the longitudinally split 

 chromosomes) now move apart toward the poles of the achromatic 

 figure, where they soon form two closely packed groups with the central 

 spindle of connecting fibers extending between them. The period during 

 which the daughter chromosomes are thus moving apart is known as the 

 anaphase. The two groups of daughter chromosomes now reorganize 

 the daughter nuclei, in each of which the chromosomes again form a 

 reticulum like that of the original mother nucleus. This reorganization 

 period is called the telophase. During the telophase there is formed 

 upon the connecting fibers (central spindle) a separating wall, which 

 completes the division of the cell. The nucleolus as a rule plays no con- 

 spicuous part in mitosis: it usually disappears during the late stages of 

 the prophase, new nucleoli being formed in the daughter nuclei in the 

 telophase. In rapidly dividing cells the period between two successive 

 mitoses is called the interphase. 



Mitosis in animals (Fig. 49) is closely similar to that in plants as 

 regards the behavior of the chromosomes. It normally differs in two 

 conspicuous features, namely, the presence of centrosomes and the 

 mode of cytokinesis following the division of the nucleus. 



During the prophases the centrosome with its aster, if not already 

 double, divides. The two daughter centrosomes, each with its own 

 aster, move apart, and a small bundle of fibers extends between them; 

 all these structures together form the amphiaster. The rays on the side 

 toward the nucleus extend into the latter when the membrane dissolves 

 and become attached to the chromosomes, often before the two centro- 

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