PROTOZOA 19 



They usually contain masses of some size composed of various 

 materials. Such masses, when they consist only of the substance 

 known as plastin (which takes acid stains), are known as nucleoli \ 

 if they also contain chromatin (basic-staining) they are amphi- 

 nucleoli. A single central mass is an endosome : it may be a temporary 

 aggregation, as, for instance, in Actinosphaenum, but more often is 

 permanent except, sometimes, at division. Such a permanent en- 

 dosome is usually a nucleolus or an amphinucleolus, but is said some- 

 times to consist solely of chromatin or of achromatic matter. A 

 permanent endosome consisting of plastin or chromatin, or both, is 

 known as a karyosome. Two principal types of protozoan nuclei — the 

 dense and the vesicular — may be distinguished; there are, however, 

 intermediates between them, and they do not characterize each a dis- 

 tinct branch of the phylum, but the dense appears to have arisen 

 more than once from the vesicular. In nuclei of the dense type the 

 achromatic part has a relatively firm consistency, and often exhibits, 

 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. 86-89). 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. 

 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 

 process. The division centre may be intranuclear or extranuclear ; 

 when it is an extranuclear centriole, it is often identical or associated 

 with the basal granule of a flagellum. Cases in which the chromosomes 

 are distinct and on the whole behave like those of metazoa are known 

 as eumitoses. Another set of mitoses, known d.s paramitoses (Fig. 19), 

 differ from those of the Metazoa in that the chromosomes do not 

 shorten in the metaphase, and are not symmetrically arranged on the 

 equator of the spindle (if such be visible); and their longitudinal 

 halves, when they separate, hang together to the last at one end so 

 that they appear, though deceptively, to divide transversely. In a 

 third set, known as cryptomitoses (Fig. 20), there are no distinct 



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