20 



CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



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, the 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 convolu- 

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

 connecting loops, forming the wreath. The nuclear membrane and the 

 nucleolus disappear, the latter passing at times into the cell protoplasm and 

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

 chromosomes, of which each species of animal has a constant and character- 

 istic 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. 



A B 



FIG. 15. Leuco- 

 cyte of Salamander 

 Larva, Showing At- 

 traction Sphere. 

 (After Flemming.) 



FIG. 16. Early Stages of Karyokinesis. A. The thicker primary fibers remain and 

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

 spindle becomes longitudinal. (Waldeyer.) 



The 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 



