256 l^HE PROTOZOA 



linin reticulum occurs around the central karyosome, the history of 

 which has been recorded by Schaudinn ('96) in Actinophrys and 

 Acanthocystis, arrd by Brauer, Gruber, and Hertwig in Actinospfuzrium. 

 In the latter form, which may well serve as a type, the karyosome in 

 nuclei of ordinary vegetative forms is distinctly granular, the chromatin 

 granules being grouped in a variety of ways. There is a constant 

 tendency, however, for the granules to extend outward along the 

 course of the linin reticulum, and toward the nuclear membrane, 

 while during mitosis the granules are grouped together in the centre 

 of the nucleus and along parallel lines. 



In nuclei with from ten to twelve karyosomes, the history is prac- 

 tically the same as in the simpler cases with one. Here also each 

 karyosome breaks down into numerous granules, but there is no linin 

 reticulum, and the granules unite in lines to form the chromosomes 

 (Noctiluca t Figs. 138, 141). The formation of the numerous granules 

 takes place by division of the karyosomes instead of by budding as in 

 the Sporozoa. 



Another type of nucleus is characterized by small-sized chromatin 

 granules, which may or may not be strung upon linin threads. 

 Familiar examples are seen in Euglena viridis and the majority of 

 the flagellates. Here the chromatin is in the form of small rod-like 

 granules connected by linin threads and surrounding a central division 

 centre, all being inclosed within a nuclear membrane (Fig. 136). 

 Upon division, the rods of chromatin separate into two groups. 

 Neither the observations of Blochmann nor of Keuten show any indi- 

 cation of splitting of the granules. In allied forms the nuclear mem- 

 brane may be either present or absent ; when absent, the granules 

 of chromatin may permanently surround a central body (Chilomonas, 

 some trachelomonads), or may be scattered throughout the cell, 

 collecting only for division (Tetramitus, Urostyla}. In Tetramitus 

 the granules collect, without fusing, about such a centre, but in Uro- 

 styla the numerous portions of the macronucleus fuse into a single 

 nucleus for division (Balbiani, '60; R. S. Bergh, '89). 



Among the Protozoa as a whole, there is no general agreement in 

 the chromatin changes preliminary to division. The object of such 

 changes, as Roux first pointed out, is apparently to get the chromatin 

 in the best position for equal division, 1 and the formation of granules 

 from the larger masses is a widely spread if not universal phenomenon 

 leading to this end. Three well-marked types of granule formation 

 have been described (i) by Schewiakoff {Eiiglyphd}; (2) by Gruber, 

 Brauer, and Hertwig {Actinosphcerium) ; and (3) by Calkins (Noctilucd). 

 In the first type the process is described by Schewiakoff ('88) as con- 

 forming to the metazoan type ; the granules form a distinct spireme 



!Cf. Gruber ('84). 



