40 NATURAL SCIENCE. Jan., 



The condition shown in Fig. 7 is called the dyaster. In 

 Figs. 5-8 only half the number of original chromosomes have been 

 drawn. The last phase (Fig. 9) is marked by the disappearance of spindle, 

 centrosomes, archoplasmic spheres, etc., and a return to the resting 

 condition, accompanied by division of the body of the cell. At the 

 moment when, in the dyaster stage, the archoplasmic spheres have 

 attained their maximum development, the division of the cell-body 

 commences. This usually takes place by a nipping of the cell trans- 

 versely to the axis of the spindle. The furrow grows deeper and 

 deeper inwards, until it cuts the cell into two nearly equal halves. 

 As soon as this is accomplished, the archoplasmic spheres begin to 

 disappear, and the chromosomes rapidly pass into the restmg 

 condition, apparently by imbibition of fluid. This stage {Fig. 10) has 

 been termed the dispirem The chromosomes eventually break up 

 into small grains, and a vesicular nucleus is formed, with the 

 chromatin substance distributed as a network or coiled thread. This 

 passage of the chromosomes from an active to a passive condition 

 has been followed out most carefully by Boveri, who has shown that 

 they preserve their individuality throughout the whole of the resting 

 stage. The nuclear membrane is apparently formed by a con- 

 crescence of some of the small grains of chromatin into which the 

 chromosomes have broken down. 



The above facts may be looked on as being typical of all forms 

 of karyokinesis, as far as regards the behaviour of the chromosomes ; 

 in the cells of different animals, however, these may vary con- 

 siderably in number and appearance, from a few large threads, as 

 in the above instance, to a large number of more or less irregular 

 granules. Furthermore, although an exact splitting of each chromo- 

 some of the nucleus of the parent cell has not been definitely 

 observed in all cases, especially where the chromosomes are small and 

 numerous, it may be almost certainly assumed that some sort of 

 process of this kind invariably does take place, either during the 

 resting stage or when the cell is undergoing karyokinetic change. 

 Before going on, however, to more debatable ground, where not only 

 the interpretation of the facts, but even information as to the facts 

 themselves, varies as we study the works of one author or of another, 

 certain abnormalities observed in karyokinesis deserve a passing 

 mention. It has been found that cell-division can be influenced in a 

 remarkable way by varying the external conditions under which it is 

 placed. The brothers Hertwig have given an account of some 

 experiments performed on the eggs of the sea-urchin, Stvongylocentnis, 

 of which the following are the most remarkable. 



Intense cold, e.g., 2° C, has been shown to destroy all parts of the 

 nuclear figure excepting the chromosomes. Dilute quinine appears 

 to have an extraordinary effect on the centrosome, as a result of 

 which it divides into four or five, each surrounded by its archoplasmic 

 sphere, and consequently the figure may have four or five polar stars 



