54 GENERAL EMBRYOLOGY 



themselves seem to be entirely passive, except in a very few 

 isolated instances where they are definitely amoeboid (Opalina) 

 (Fig. 31). The two chromosome groups are finally drawn 

 completely to the opposite poles of the spindle and the process 

 of mitosis then enters upon its final period, the telophase. 



During this phase the cytoplasmic portion of the cell becomes 

 divided into two parts, usually equal, though occasionally 

 extremely unequal. Sometimes, as in many animal cells, this 

 division of the cytoplasm results from its peripheral constriction 

 in a plane corresponding with that of the equatorial plate, the 

 constriction deepening until the cell body is completely severed 

 and the two daughter cells formed (Figs. 20, L; 21, /, J; 22, F). 

 More frequently, in some animal and in practically all plant cells, 

 the division of the cytoplasm results from the formation of a 

 partitioning cell wall, in the formation of which the interzonal 

 fibers of the spindle usually take an important part. These seem 

 to increase in number and to thicken in the middle, ultimately 

 fusing and forming a transverse plate which is the rudiment of 

 the future cell wall ; the remainder of the wall forms as a distinct 

 secretion of the cytoplasm in that region. 



As the diverging chromosomes approach the poles of the 

 spindle they lose their distinct outlines, become vesicular, and 

 gradually lose their visible identity and separateness to a large 

 extent; with a few exceptions they finally seem to disintegrate 

 completely and form scattered granules, and a new typical 

 nucleus is constituted in each daughter cell. Meanwhile the 

 centrospheres and asters have diminished in extent and clear- 

 ness and have returned again to the condition which is char- 

 acteristic of the interkinesis. About the new nucleus a mem- 

 brane is formed, either from the nucleus or the cytoplasm, and 

 the mitosis is accomplished (Figs. 20, L; 21, H-J). In most 

 cases just about this time the centrosome divides in prepa- 

 ration for the next mitosis. During the interkinesis the nucleus 

 and cytoplasm increase in size and soon the process of division 

 is repeated. 



The length of time occupied by the whole process of mitosis 

 varies greatly. In the division of some egg cells it may be 



