400 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the nucleus is formed on the "massive" type; that is to say, the 

 nuclear membrane incloses a thick mass of nuclear substance, which, 

 in all probability, contains the chromatin in the form of small 

 granules. So far, the nucleus of the Cilio-flagellata resembles that of 

 the next group — the Ciliata. 



The description of the nuclei of the Ciliata offers considerable 

 difficulties in consec[uence of the numerous variations which are to be 

 seen in the structure of even closely allied species. The nuclear 

 substance may be so dissolved in the cell- substance, and the granules 

 may be so fine as to be only distinguishable with the highest powers ; 

 or the constituents of the nucleus may be larger, and formed (as in 

 OxytricJict) of spherical corpuscles which, before division, unite into 

 a mass. This substance may form bands and plexuses, and sometimes, 

 as in Benedenia and Plagiotoma, break up into pieces ; this leads to 

 the rosette-like nuclei of Stentor, or the band-like nuclei of Vorticella. 

 It is rare for an Infusorian to have more than one nucleus, but the 

 number of the paranuclei is by no means so constant. The nucleus 

 is generally " massive " and surrounded by a membrane ; its sub- 

 stance is very rich in chromatin-granules, which are very variously 

 arranged ; the paranuclei are likewise massive, and apparently always 

 granular. On division, the chromatin-granules form filaments which 

 lie parallel to the long axis of the nucleus, and become constricted in 

 the middle. 



The nucleus of the Suctoria, the last group of all, is either 

 branched or rounded ; there is a thick massive nuclear substance, in 

 which chromatin-granules are often very distinctly visible. On division, 

 the nuclei break up into filaments which undergo constriction. 



The author thus sums up the results of this important investi- 

 gation : — 



There are Protozoa in which the nuclear substance may be dis- 

 tributed through the protoplasm of the cell in the form of numerous 

 granules ; and these are often so small that after staining they only 

 appear, on examination under high powers, as a precipitate. In others 

 there are nuclear particles of this kind, but they are not only more 

 numerous, but are also larger, and, in fact, more regularly arranged, 

 so that tbey may be better spoken of as small nuclei ; these lead us 

 to the truly multinucleate forms. He thinks it possible that in those 

 Protista which appear to us to be non-nucleate, the nuclear substance 

 is more or less completely dissolved in the cell-substance ; and that 

 in the history of race development there was not at first a definite 

 and formed nucleus, but rather fine nuclear granules. In any case, 

 the formation of a true nucleus is intimately associated with the 

 process of reproduction, and, primarily, with regular division. 



A most important piece of evidence is afforded by those Protozoa 

 which, after conjugation and division, are for a time filled with small 

 nuclear particles. It would appear that there is a regular distribution 

 of the chromatin in the daughter-individuals. 



The nuclei of the Protozoa belong, as a rule, to one of two types : 

 they are either vesicular, as in most Rhizopods, Heliozoa, Sporozoa, 

 and all true Flagellata, as well as in some Radiolaria and Ciliata, or 



