KARYOKINESIS. 185 



insufficient grounds, and most other examples which have been adduced have also 

 failed when subjected to a rigorous investigation. 1 



While, therefore, we are not in a position to deny the occasional occurrence of 

 amitotic nuclear division, it must yet be affirmed that in by far the largest number of 

 cases in all classes of plants and animals cell-division takes place by the mitotic 

 process. 



Karyokinesis. Indirect or mitotic division of the nucleus. The following 

 is a brief account of the more important phases of karyokinesis which have been noticed 

 in typical instances of indirect or mitotic division in animal cells : 



The nucleus of a cell which is about to divide first becomes somewhat enlarged 

 and its chromoplasm loses the net-like arrangement which is usually met with in resting 

 nuclei, and becomes converted into a closely constricted skein of filaments (fig. 211, b). 

 There is some difference of opinion as to whether at this " skein " stage there 

 are a number of separated filaments, or whether there is one long filament only. Both 

 the nucleoli and the chromatic nuclear membrane become merged into the skein of 

 chromoplasm, and the outline of the nucleus against the cell-protoplasm becomes in 

 consequence less distinct. According to Rabl, the conversion of the network into the 

 skein is effected by the withdrawal of all the secondary anastomosing chromoplasmic 

 filaments into the primary convoluted loops (fig. 209, C) : the latter therefore become 

 thicker arid more evident, and this process of thickening accompanied by shortening 

 of the convolutions proceeds until a far less complex and more open skein is the result 



Fig. 212. APPEARANCE OF ACHROMATIC SPINDLE IN POLAR AREA OF DIVIDING NUCLEUS. (Rabl.) 

 Fig. 213. MOVEMENT OF SPINDLE TOWARDS THE MIDDLE OF THE NUCLEUS, ACCOMPANIED BY THE V-SHAPED 



CHROMOSOMES, SOME OF WHICH ARE ALREADY SPLIT LONGITUDINALLY. 



(fig. 211, d), in which the constituent looped filaments can nowbe seen and their arrange- 

 ment relative to pole and antipole of the nucleus made out (fig. 209, A and B). At 

 about this stage of the process of karyokinesis, a spindle-shaped system of achromatic 

 fibrils which is known as the achromatic spindle, makes its appearance at the pole of 

 the nucleus, either taking origin altogether within the nucleus, or arising partly or 

 entirely in the adjacent protoplasm and passing into the nucleus at its polar area (figs. 

 212 and 213). At any rate the poles of the spindle soon reach as far as or even project 

 beyond the limits of the nucleus on either side, and the achromatic fibres which com- 

 pose it diverge at either pole from a central particle (fig. 214), the pole-corpuscle, 

 and from this corpuscle on the other hand fibrils radiate outwards into the protoplasm 

 of the cell. 



As just stated, the achromatic spindle makes its appearance at the polar end of the 

 nucleus, towards which the loops of the chromoplasmic filaments are directed. 

 Here the spindle is placed obliquely across the polar area, and hence it moves 

 gradually towards the middle of the nucleus, where it eventually takes up its position. 

 In this movement it is accompanied by the chromatic filaments (chromosomes of 



1 Instances of amitotic division which have been described in recent years by various authors, will 

 be found referred to in Waldeyer's article on Karyckinesis, in the Quarterly Journal of Microscopical 

 Science, vol. xxx., 1889 ; also in an article by Flemming on the Division of Leucocytes, in the Arcliiv f. 

 mikr. Anatomic, Band xxxvii., 1891. 



