NUCLEAR DIVISION IN PROTOZOA. 



213 



with the centrosomes in the spheres, and then separate in the 

 lines of previous longitudinal division. At no time is there 

 a spireme, as in ordinary mitosis. 



During early stages of nuclear activity the sphere divides 

 into two similar halves connected by a strand composed of 

 fibers which are formed from the substance of the sphere. 

 These fibers compose the central spindle and are homologous 

 in every way with the central spindle fibers of the first type of 

 mitosis given above (Fig. i, A). The nucleus then elongates in 

 a direction at right angles to the central spindle, and at the 

 same time it bends in the center in such a way that the cen- 

 tral spindle sinks into a depression in the nucleus which sur- 



' "■ ^^^ ■' 





% 



"I 



Fig. I. — Stages in mitosis of Noctiluca miliaris. A , amphiaster and chromosome formation ; 

 B, metaphase sliovving central spindle in the hollow of the nucleus; C, section through 

 long axis of the central spindle ; U, section through one pole of late anaphase in spore- 

 forming mitosis. Double centrosome connected with the longitudinally divided chromo- 

 somes by mantle fibers. 



rounds it upon three sides (Fig. i, B). In this way the nuclear 

 plate is finally wrapped about the central spindle in the form 

 of an incomplete ring, a condition, it will be observed, which 

 obtains in all higher mitotic figures where the central spindle 

 is present. The nuclear membrane then disappears in that 

 part of the nucleus which is turned towards the central spindle, 

 while it is retained unbroken in all other parts of the nucleus 

 (Fig. I, C, D). Thus the chromosomes again, as in the higher 

 type, are brought in contact with the central spindle fibers. 

 They then split longitudinally, and, through the agency of 

 mantle fibers, are separated into two equal groups, each group 

 drawn towards its own daughter-sphere. Within the sphere 

 the mantle fibers are focused in a centrosome which at this 

 period can be demonstrated with the greatest ease (Fig. i, D). 

 The division is finally completed by the separation of the 



