24 



THE INVERTEBRATA 



at least in fixed specimens, a fine meshwork. The plastin is in masses 

 scattered through the nucleus, or occasionally in a single excentric 

 mass. The shape is often not spherical (Figs. 87-90). The meganuclei 

 of ciliophora and dinoflagellate nuclei belong to this type, which 

 otherwise is rare. In vesicular nuclei the achromatic part is more fluid 

 and its meshwork, if any, is coarse. The plastin may be in several 

 masses under the nuclear membrane, but usually is in a karyosome. 



spd. 



.-cen. 



nn.mr.^ 



Fig. 19. Mitosis (paramitosis) of the sporogony oi Aggregata eberthi. After 

 Belar. A, Interphase between divisions. B, Early metaphase. C, Anaphase 

 beginning. D, Later anaphase. E, Early telophase. F, Later telophase. 

 cen. centriole; chr. chromosomes; 7iu.me. nuclear membrane; spd. spindle. 



The modes of division of protozoan nuclei are also very various. 

 Many were formerly classed as amitoses but are now regarded as un- 

 usual types of mitosis. True amitoses are rare, and perhaps occur only 

 in the meganuclei of the Ciliophora. The mitoses are sometimes (Fig. 

 58) practically identical with those of the Metazoa, but are usually 

 more or less aberrant. The "division centre" by which mitosis is 

 initiated may be a centrosome consisting of centrosphere and 

 centriole, or may be either of the latter two entities alone. The 

 centrosphere often forms a plate or cap at each pole of the nucleus. 

 Most often the nuclear membrane remains intact throughout the 



