No. 3] THE PROTOZOA AND METAZOA. 719 



some Metazoa, may be called the " central-spindle " (Figs. 9 

 and 11). The central-spindle then sinks into the nucleus 

 which forms a trough to receive it (Fig. 13). The trough 

 deepens and the lips of the nucleus approach each other over 

 the central-spindle until the latter finally occupies the position x 

 of the Fig. ©, the daughter-spheres lying partly in the groove 

 and partly exposed at the ends (Fig. 13). The nucleus then 

 divides in a plane at right angles to the central-spindle, 

 each half being accompanied by one of the daughter-spheres. 

 The anaphase-stage presents the well-known striated appear- 

 ance of protozoan mitotic figures — the striations being formed 

 by the daughter-chromosomes which are directed towards the 

 separating spheres (Fig. 18). In the final stage of division 

 the daughter-nuclei become completely separated, the furrow 

 in which the central-spindle had lain becomes obliterated, and 

 the sphere resumes its normal appearance in the resting cell. 



In spore-forming divisions the process is in the main the 

 same, but the mitoses follow each other in quick succession 

 and without intervening resting stages. Here the daughter- 

 chromosomes of an anaphase without further change form the 

 chromosomes of the ensuing prophase. The spheres again 

 divide and the process is continued until the original nucleus 

 is divided into as many parts as there will be spores (from 300 

 to 600). 



I. The CJu'omatin. 



a. Prophase. — Nuclear division is preceded by a concentra- 

 tion of the cytoplasmic microsomes in the sphere. The hyaline 

 area disappears, the entire mass diminishes in size and becomes 

 more homogeneous, and by these changes the sphere becomes 

 more dense and more clearly defined, so as to be even more 

 conspicuous than the nucleus. It was probably this stage 

 which misled Brightwell ('57) and Robin ('78) into mistaking 

 it for the nucleus. 



Within the nucleus, meantime, the large basichromatic 

 karyosomes gradually break up into smaller pieces, apparently 

 in the same way as the chromatin in Actinosphaeriiim accord- 

 ing to Gruber's ('83) and Brauer's ('94, '94a) descriptions. The 

 disintegration of the karyosomes seems to be accomplished by a 



