No. 3.] THE PROTOZOA AND METAZOA. 725 



2. The Nuclear Membrane. 



In the resting cell the nuclear membrane is comparatively 

 distinct and thick, but in the active nucleus it becomes very 

 much thinner and more plastic. Except in one region it does 

 not disappear during mitosis, but persists as a permanent 

 portion of the nucleus. In one region, however, it does dis- 

 appear during mitosis. When the nuclear plate is formed and 

 the chromosomes are ready for division, it disappears in the 

 region between the nuclear plate and the central-spindle. The 

 nuclear plate is thus left as a free ring around the central 

 spindle (Figs. 13, 29, 31). As the daughter-chromosomes 

 separate, the membrane is reformed between them (Fig. 30), 

 but it always remains broken in the region between the 

 daughter-chromosomes and the spheres. The position of the 

 gap which is thus formed in the membrane can be clearly seen 

 from Fig. 31. Here the membrane is complete in all places 

 except where the mantle-fibers pass from the centrosomes 

 to the chromosomes. During the telophase the membrane 

 reforms in the broken places and again becomes continuous. 



The above-described gap in the membrane during mitosis is 

 strikingly similar to a temporary stage in metazoan mitosis 

 which has been described by many observers. In all such cases 

 the nuclear membrane disappears first at the poles in front of 

 the developing spindle-fibers. In some instances (e.g., in Thal- 

 lassema Grififin ('96) ) the membrane persists in this stage for 

 some time, and only at a late period in mitosis does it completely 

 disappear. 



3. The Sphere. 



We may now consider the history of the sphere more in 

 detail. As previously stated, the approach of division is first 

 recognized by the concentration of cytoplasmic microsomes in 

 the sphere. During disintegration of the karyosomes and 

 formation of the chromosomes a structure is formed consisting 

 of two daughter-spheres connected by a "central-spindle," to 

 which, from analogy with metazoan mitoses, the name of 

 amphiaster may be given (Figs. 9, 11). As the nucleus elon- 



