THE REPRODUCTION OF THE PROTOZOA 117 



plates lying within the persistent membrane. External to the 

 membrane are two large conical masses of archoplasm, termed the 

 " polar cones." As in the micronucleus of Paramecium, the polar 

 plates represent functionally the centrosomes, towards which the 

 daughter-plates travel, and division of the nucleus is effected by 

 growth of the separation-spindle. The archoplasmic polar cones 

 appear to take little or no part in the mechanics of the division, 

 since their apices maintain their distance from one another, and 

 the growth of the separation-spindle pushes the daughter-nuclei 

 into their substance. 



The reduction-karyokinesis is heralded by the formation of a 

 centrosome from the nucleus (Fig. 37 ; see p. 80, supra). The 

 centrosomes are at first close to the nucleus, external to its mem- 

 brane, but when the karyokinetic spindle is formed the centro- 

 somes travel to the apices of the cones. From the centrosomes 

 radiations extend through the polar cones, continuing the direction 

 of the longitudinal striations of the intranuclear spindle, though 

 separated from them by the intervening nuclear membrane. During 

 the division the apices of the cones move apart to a slight extent, 

 but the separation of the daughter-nuclei is still mainly the work 

 of the separation-spindle, which pushes them into the polar cones 

 and brings them close to the two centrosomes again ; hence the 

 activity of the polar archoplasm can be but slight. The chromo- 

 somes in the reduction-divisions are more distinctly separated from 

 each other as the result, apparently, of a reduction in the amount of 

 the plastin forming the ground-substance. The nuclear membrane 

 persists throughout the whole process. 



In Actinophrys the karyokinesis appears to be of a type similar 

 to that of Actinosph cerium, with persistent membrane, but with 

 more activity in the extranuclear archoplasmic elements. In 

 Acanthocystis (Fig. 64), however, the nuclear membrane disappears 

 completely from the karyokinetic figure, and it is no longer possible, 

 in consequence, to distinguish the parts of the achromatinic spindle 

 which are of intranuclear and extranuclear origin respectively. 

 Nuclear and cytoplasmic elements are in complete co-operation, a 

 condition of things which has apparently been brought about and 

 rendered possible by the extrusion of the centrosome from the nucleus 

 in the first instance. 



From the foregoing examples, it is seen that amongst the Protozoa 

 the material is to be found for illustrating the gradual evolution of 

 the mechanism of karyokinetic division, from the starting-point of 

 simple and direct division up to the most advanced type in which 

 a perfect karyokinetic figure is formed by co-operation of nuclear 

 and cytoplasmic substance. It is not necessary to suppose, how- 

 ever, that the course of evolution has always been in the direction 



