114 



THE PROTOZOA 



amount of chromatin increases, and the nucleus becomes oval in 

 form. The chromatin forms a number of chromosomes shaped like 

 elongated rods or short threads, which arrange themselves at the 

 equator. At the same time the achromatinic framework shows a 

 longitudinal fibrillation or striation, the apparent fibrillae being 

 centred in thickenings of the achromatinic framework which appear 

 at the two poles of the nucleus within the persistent nuclear mem- 

 brane, hence termed the " polar plates." Centrosomic grains are 

 stated to be entirely absent, and their functions are performed by 

 the polar plates. The nucleus continues to elongate, and the 

 chromosomes divide transversely to their long axis to form the 

 daughter-plates, which travel apart ; as they do so the fibrillated 



B 



G 



FIG. 61. Stages in the division of the micronucleus of Para- 

 mecium. A, B, Early stages ; O, spindle-stage with equa- 

 torial plate of chromosomes ; D, spindle with the two 

 daughter-plates; E H, growth of the separation-spindle 

 and separation of the two daughter-plates ; /, rcconstitu- 

 tion of the daughter-nuclei, whioh are widely separated, 

 but still connected by the greatiy elongated separation- ^ 

 spindle, the central part of which shows a dilatation prior to its final 

 absorption. After Hertwig. Figs A E are drawn on a larger scale than 

 the other figures. 



/ 



separation-spindle appears between them. The nucleus as a whole 

 now becomes dumb-bell-shaped ; the daughter-plates are lodged in 

 the terminal swellings, while the rapidly-growing separation-spindle 

 occupies the handle of the dumb-bell. The daughter-plates now 

 break up and reconstitute the daughter-nuclei, but the connecting 

 portion continues to elongate and to push the daughter-nuclei apart. 

 It is clear that the separation is effected by intrinsic growth of the 

 achromatinic framework constituting the separation-spindle, which 

 is often curved up into a horseshoe-figure, and shows bending or 

 twisting of its fibrils, as the result of the inert resistance of the sur- 

 rounding cytoplasm. Finally, however, a limit of growth is attained ; 

 the daughter-nuclei become constricted off completely from the 

 connecting bond, which is absorbed and disappears. The nuclear 

 membrane persists throughout the division. 



In all the forms of nuclear division dealt with so far, nuclear 

 elements alone have been active in the process. A most instructive 

 series, showing how extranuclear elements come to collaborate in 



