CELL MULTIPLICATION 



19 



plication takes place by two recognized methods, direct (amitosis), in which 

 there is little disturbance of the nuclear network, and indirect (mitosis), in 

 which there is a complex series of nuclear network changes. These mitotic 

 changes result in the division of the chromatin fila- 

 ments into the two new parts which form the chromo- 

 plasm of the daughter nuclei. 



The process may be divided into the following 

 stages: 



Prophase. The resting nucleus becomes somewhat 

 enlarged, and the centrosome (according to those who 

 regard it as lying normally within the nucleus) migrates 

 into the cell protoplasm. The centrosome then divides 

 into two daughter centrosomes which lie near the nucleus 

 but are separated by a considerable interval. Each is 

 surrounded by the radiating fibrils of the attraction 

 sphere, and some of these fibrils pass continuously from 

 one centrosome to the other, forming the achromatic 

 spindle. At the same time the intranuclear network be- 

 comes converted into a fine convoluted coil (spirem or 

 skein) which may be either continuous or else broken up into several threads. 

 The thread or threads then shorten and become thicker, while the convolutions, 

 which have become less numerous, arrange themselves in a series of con- 

 necting loops, forming the wreath. The nuclear membrane and the nucleolus 

 disappear, the latter passing at times into the cell protoplasm and disintegrat- 

 ing. The wreath then breaks up into V-shaped segments, the chromosomes, 

 of which each species of animal has a constant and characteristic number. 

 This varies in the different animals, but is sixteen in man. 



The two centrosomes migrate to the poles of the nucleus, while the achro- 

 matic spindle which connects them occupies the long axis of the nucleus. The 



FIG. 15 Leucocyte 

 of Salamander Larva, 

 Showing Attraction 

 Sphere. (After Flem- 

 ming.) 



FIG. 1 6. Early Stages of Karyokinesis. A. The thicker primary fibers remain and the achro- 

 matic spindle appears. B. The thick fibers split into two and the achromatic spindle becomes 

 longitudinal. (Waldeyer.) 



chromosomes, becoming much shorter and thicker, gather around the spindle 

 in its equatorial plane, with their angles directed toward the center, forming 

 the aster or monaster. 



Metaphase. The actual division of the nucleus is begun at this time by the 

 splitting of each chromosome longitudinally into halves which lie at first close 



