THE PHENOMENA OF MATURATION IN PROTOZOA 167 



tion process. Its work is done, and, like the somatic cells of the 

 metazoa, it dies. The micronucleus, on the other hand, after lying- 

 dormant so far as the vegetative functions of the cell are concerned, 

 now begins its germinal activity. It moves away from the macronu- 

 cleus, where it usually lies in a cleft in the substance of the macro- 

 nucleus, and begins to swell. It contains two substances: one, located 

 at one pole of the nucleus, is the substance of the division centre, and 

 gives rise to the fibers of the spindle figure, so that in it rests the poten- 

 tial energy which is later converted into the kinetic energy of division. 

 The other substance is chromatin, which is concentrated at this time 

 in a number of granules closely packed against the division centre. 

 The nucleus then elongates by fragmentation of the chromatin gran- 

 ules, the fragments arranging themselves in lines radiating out from 

 the division centre. They correspond to the idiochromidia of the 

 rhizopod cell, but are now assuming definite form, the irregular and 

 distributed idiochromidia of the more primitive organisms being- 

 replaced here by the more definite chromosomes. The elongation of 

 these lines of chromatin continues until the nucleus is an enlarged, 

 narrow structure many times longer than the resting nucleus. The 

 intranuclear division centre, which is concentrated at one end of the 

 nucleus, likewise increases in size (Fig. 72). 



The micronucleus next becomes curved in such a way that the two 

 ends are brought close together, forming a distinct crescent, with the 

 long lines of chromatin uniting to make a branched network extending 

 from tip to tip, while the division centre, now much enlarged, moves 

 toward the centre of the crescent. The chromosomes of the first 

 division figure are formed by the transverse division of the elongated 

 lines of chromatin granules, but, owing to the net formation and 

 association side by side, these short fragments are each double, a 

 longitudinal split appearing in each. All of the chromatin is thus 

 utilized and an uncountable number of chromosomes are thus formed. 

 The substance of the division centre then diffuses through the nucleus 

 in a kind of flowing division and the two poles of the first maturation 

 spindle are formed by the accumulation of this material at the opposite 

 sides of the nucleus. With this flow the chromosomes are divided, so 

 that when the spindle is entirely formed the daughter halves of the 

 chromosomes are separated and now lie end to end in the so-called 

 anaphase stage of division (Fig. 72, C). (See Calkins and Cull ('08) for 

 the details of this spindle formation.) 



The nucleus then divides by constriction through the middle and the 

 first two maturation nuclei are the result. Each of these next divides 

 again, the process of division being identical with that described 

 above and four maturation nuclei are formed. Two of these immedi- 

 ately begin to degenerate, while a third follows suit shortly after, the 

 fourth alone dividing a third time. Here the chromatin is not divided 



