22 



CYTOLOGY CHAP. 



from the linin ; and finally the whole achromatic figure, including the 

 centrosomes, may be intranuclear {e.g., Ascaris megalocephala univalens, 

 Brauer, 1893). 



In nearly all resting cells the achromatic figure disappears except for 

 the centrosome, and occasionally a mass of differentiated cytoplasm 

 surrounding it, called the attraction sphere ; this corresponds to the 

 central mass of the aster which surrounded the centrosome during mitosis. 

 Even the centrosome can only be demonstrated on favourable material. 

 In most resting cells both cytoplasm and chromatin seem to be disposed 

 without reference to the centrosome, but in others this body obviously 

 exerts a powerful influence on the disposition of the various cell con- 

 stituents. A good example of such a cell is afforded by the gametocytes 

 in the " bouquet " stage (Chapter II.). 



Though so minute, the centrosome is often a conspicuous body owing 

 to its intense affinity for certain common stains. In order to form the 

 spindle figure it divides by simple fission into two daughter centrosomes, 

 which separate from one another, spinning out the central spindle (when 

 such is present) between them, and each becoming the centre of an astral 

 radiation. These facts have led many cytologists to look upon, the 

 centrosome as a permanent cell organ, comparable in autonomy to the 

 nucleus, and only arising by division of a previous centrosome. This 

 view, however, is beset with grave difficulties. There is, for instance, 

 ' strong evidence that a centrosome may arise de novo in the cytoplasm, 

 and thereafter behave in precisely the same way as a centrosome derived 

 by fission from a previous centrosome (p. 95). Moreover, in the higher 

 plants, which have an achromatic figure otherwise essentially hke that 

 of animals, there are no centrosomes. 



The division of the centrosome may, like that of the chromosomes, 

 take place in anaphase, telophase or prophase. In the two former cases 

 it is of course double in the resting cell. 



It must be remembered that the division of the chromosomes is an 

 autonomous process independent of the achromatic figure, for it often 

 takes place before the spindle figure is formed or while it is still outside 

 the nucleus. For the separation of the daughter chromosomes, however, 

 a properly developed achromatic figure appears to be essential. Thus 

 Wilson (1901) found that in the eggs of the sea-urchin developing by arti- 

 ficial parthenogenesis (p. 95), various abnormahties of the achromatic 

 figure often appeared. One such irregularity was the failure to form a 

 proper bipolar spindle, instead of which a single aster only was formed 

 (monaster, as opposed to the amphiaster of a normal bipolar figure). In 

 such eggs the ordinary nuclear cycle may be gone through many times. 

 At each mitosis the chromosomes divide, but the daughter halves do not 

 separate ; instead of forming two daughter nuclei, they enter into a 



