KARYOKINESIS. 



system of fibrils, which v. Beneden terms the polar cone (fig. 219, p.c.}. Besides these 

 fibres of the spindle and its polar cones, other striae radiate from the attraction-spheres 

 into the surrounding protoplasm, leaving a somewhat prominent equatorial zone of 

 cell-protoplasm free from radiations, and marked off from the radially striated part 

 by a distinct boundary line. After the formation of the daughter-nuclei, each pole- 

 corpuscle, with its surrounding attraction-sphere, is described as undergoing division 

 and remaining within the daughter-cell as a double system of spheres, ready to 

 initiate the further division of the nuclei which have just been formed. If this be 

 so the attraction-spheres are not to be regarded as developed only during karyokinesis 

 but as forming a prior constituent of the cell, merely becoming more evident at the 

 time of cell-division, and along with the achromatic spindle, which subsequently 

 unites them, may be regarded as initiating and directing the division of the nucleus 



Fig. 220.- 



-STAGES IN THE DIVISION OP THE NUCLEUS OF A LIVING EPITHELIUM CELL IN THE EPIDERMIS 



OP A SALAMANDER-LARVA. (Flemming.) 



a, cell showing the nucleus transformed into a mass of contorted filaments (corresponds to 5 and c of 

 fig. 211) ; b, the nuclear filaments have become fewer and more united, and begin to assume a con- 

 verging arrangement (compare d, e, /, of fig. 211) ; c, stellate form (compare y, fig. 211) ; d, e, hour- 

 glass contraction of the nucleus which gradually passes off and the form c is resumed. This may occur 

 more than once : eventually the filaments accumulate in a direction parallel to one another near the 

 centre of the cell, and then gradually separate into two sets as shown in/ (i, j, k in fig. 211). These as 

 they retire towards the poles gradually resume the stellate form r/ (I, fig. 211). The time occupied 

 whilst the stages above represented were passed through was about three hours. 



and of the cell. In the resting condition of the cell the attraction-sphere and central 

 particle are single, but they undergo division at the commencement of karyokinesis 

 (Boveri). Attraction-spheres and pole-corpuscles have also been described by 

 Flemming in the white blood-corpuscle, and are probably of constant occurrence. 



The karyokinetic process has been watched in actual progress in all its stages by 

 more than one observer. The time occupied varies in different animals from half 

 an hour to three hours. Observed thus in the living cell (fig. 220) it is not possible 

 to follow out all the details of the process, which have only been elucidated in tissues 

 the cells of which have been fixed by appropriate hardening reagents and afterwards 

 stained. 



The number of the chromatin filaments or chromosomes varies in different animal- 

 and plant-cells from 2 (Ascaris megalocephala) to 24 (Salamandra maculosa) or 

 more. It is probably constant, or nearly so, in the same species, but may be very 

 different even in allied species. In the division of the spermatogenic cells of the 

 testicle to form the spermatoblasts or young spermatozoa, the final separation of the 

 chromosomes into the daughter-nuclei takes place without the occurrence of a 

 corresponding longitudinal cleavage ; the result of this is that the spermatoblasts 



o 2 



