60 



GENERAL PRINCIPLES OF ZOOLOGY 



FIG. 22. Spindle formation and 

 division of the centrosomes in As- 

 caris megalocephala (after Brauer). 

 c, centrosomes; ch, chromosomes. 



exhibits a certain arrangement, a fibrous structure in the direction of the 

 elongating nucleus. 



Indirect Cell Division, Karyokinesis. Indirect cell division, 

 karyokinesis or mitosis,' is most beautifully shown in cells, poor in chro- 

 matin, which possess a centrosome. The process is introduced by a 

 division of the centrosome (fig. 22). The daughter centrosomes migrate 

 to two opposite poles of the nucleus, which now loses its membrane and 

 becomes the nuclear spindle. The characteristics of the spindle are that 



it is drawn out into points at two poles 

 which are indicated by the position of the 

 centrosomes, while from these poles fine 

 threads, the spindle-fibres, run to the 

 centre or equator of the nucleus. These 

 fibres are in many cases certainly derived 

 from the achromatic nuclear reticulum, 

 while in others a greater or less part in 

 their formation is taken by the protoplasm 

 (fig. 22.) A debated point is the rela- 

 tions of the fibres in the equatorial plane 

 of the spindle. Do all the fibres extend 

 from pole to pole ? Do all of them end 

 in the equatorial plane, so that the spindle consists of two cones of fibres 

 separated at the equator? Or, lastly, are fibres of both kinds present 

 in the same spindle ? It would appear that differences exist in these 

 respects in different cells. 



All of the chromatin of the nucleus lies in the equator, united in the 

 'equatorial plate,' but by this must not be understood a connected mass 

 but a layer of separate bodies, the chromosomes (fig. 23, a). These 

 develop at the beginning of nuclear division by the union of the chro- 

 matin granules (which are distributed diffusely over the reticulum of the 

 resting nucleus) to strongly staining bodies, which are rarely spherical 

 or rodlike, but usually have the shape of U-shaped loops. It is of the 

 greatest theoretical significance, that their number is identical in all the 

 cells of all the tissues of one and the same species. 



The first step in the mitotic formation of the daughter nuclei is the 

 division of the chromosomes, which is usually completed in the equatorial 

 plate (division of the equatorial plate), but may be completed earlier. 

 The division is an accurate halving (fig. 23, b). The two halves of a 

 mother-chromosome, the daughter chromosomes, now travel, under the 

 influence of the spindle-fibres, towards the poles of the spindle. In this 

 way, by a splitting of the equatorial plate, the lateral plates arise, the 



