Jan., 1894. CELL-DIVISION. 39 



(4.) Theoretical considerations.^ 



The General Features of Kavyokinesis. 



In Figs. 1-8 (p. 43) the nucleus of a cell is represented in the various 

 phases through which it passes during karyokinesis. In Fig. i the 

 nucleus is in the resting condition, the chromatin being in the form of 

 a much-coiled thread. The first preliminary stage towards division 

 is marked by the thread acquiring a sharper contour, due to the 

 withdrawal of numerous small branches and processes. It then pro- 

 ceeds to break up into a number (eight m this case) of pieces, Fig. 2. 

 Each of these splits longitudinally into two exactly similar halves, 

 forming in all sixteen daughter chromosomes (Chr.), as the masses of 

 chromatin are now called. Fig. 3. The nuclear membrane at this 

 period becomes indistinct and gradually disappears, and a new body 

 comes into play, viz., the centrosome. Whatever may be its origin, 

 the centrosome, owing to the active part it takes in the next stages, 

 must be noticed at this point, though it will be dealt with in detail 

 later on. A centrosome is thought by many to be an integral part 

 of, and therefore present in, every cell during the resting as well as 

 the active state. Since, however, it is unaffected by ordinary 

 staining reagents, its presence is often difficult to demonstrate. At the 

 moment when the nuclear membrane begins to disappear, the centro- 

 some becomes surrounded by a number of radiating strands of proto- 

 plasm (Fig. 4, c), which form the so-called "archoplasmic sphere." The 

 centrosome then divides into two, each half remaining attached by 

 means of numerous threads (C. Sp.). These threads are, in this case at 

 any rate, the beginning of the future nuclear spindle, although in other 

 cases a different origin has been assigned to it, as will be seen when 

 the spindle is dealt with by itself. The next stage is represented in 

 Fig. 5. The centrosomes have greatly increased their distance from 

 one another, owing to the rapid enlargement of the spindle. The 

 chromosomes (.v.) have grouped themselves round the spindle in the 

 median plane, to form the so-called equatorial plate. They have, 

 furthermore, become doubled, each pair on itself, in such a way that 

 the angle of the bend is directed towards the spindle, while the 

 two ends point towards the surface of the cell. The next two stages 

 are shown in Figs. 6 and 7. The two daughter chromosomes of each 

 original chromosome gradually recede from one another and move 

 along the spindle, until they come to lie, one set at one pole of the 

 spindle and the other at the other. Nevertheless, they are not as 

 yet entirely separated from one another, but still remain connected 

 by delicate threads (Verbindungs Fasern, Hertwig) formed from the 

 so-called " lame intermediaire " of Van Beneden. These must not 

 be confounded with the threads comprising the spindle, from which 

 they are quite distinct. 



1 It has been found possible in this article to treat of only the first two of these 

 headings. 



