SPECIAL MORPHOLOGY OF THE PROTOZOAN' NUCLEUS 275 



rilst" which he described in the vegetative nucleus, although he recog- 

 nized a distinction between it and both the linin reticulum and the 

 chromatin. The achromatic figure in ActinospJicerium is so large as 

 compared with the nucleus of resting cells, that it is difficult to accept 

 Hertwig's view, that it all comes from the linin of the resting nucleus 

 (cf. Fig. 140, A, B, D). The writer, in a recent publication ('99), pro- 

 posed the application of Boveri's theory of archoplasm to the division- 

 centres of Protozoa ; and, applicable, apparently, to all types of the 

 division-figure in Protozoa, it may serve for the time as a working 

 hypothesis. Briefly stating this hypothesis, it was held that the 

 division-centre, consisting of archoplasm, retains its definite form 

 and size in the nucleus of many of the primitive forms (flagellates), 

 but under certain conditions may become enlarged or diffuse. Thus, 

 in Tetramitns, the division-centre is much more diffuse during the 

 resting phases of the cell than during division (cf. Figs. 134, 143); 

 and in Oxyrrhis marina Schaudinn ('96) found that the division-centre 

 becomes much enlarged when the cells are transferred to a medium 

 of less density, and conversely, when placed in a denser medium, the 

 structures become reduced in size and more definite. It was held 

 that the intra-nuclear division-centre becomes similarly diffused. 

 The nucleus of Amoeba proteus, for example, contains chromatin in the 

 form of minute granules, which are arranged about the periphery of 

 the nucleus, while the central portion is occupied by a large homo- 

 geneous mass, which can be explained as an enlarged or diffuse intra- 

 nuclear division-centre. The pole-plates, also, which are widely 

 distributed throughout the Protozoa, may be explained as a temporary 

 accumulation of this ordinarily diffuse archoplasmic substance, and thus 

 homologous with the " nucleolus-centrosome " or division-centre of 

 Etiglena, and with the centrosphere of Metazoa. 1 The substance 

 which may thus become diffused through the nucleus may also pene- 

 trate the nuclear membrane, until accumulations on the outside of the 

 nucleus result. Hertwig described such transfusion of achromatic 

 material from the nucleus of Actinosphcerium to the aggregates (Pro- 

 toplasmakegel} on the outside (Fig. 140, E). Finally, just as it be- 

 comes diffused throughout the nucleus in Protozoa and Metazoa, so 

 it may become diffused throughout the cytoplasm in Metazoa, as 

 postulated by Boveri. 



A number of ingenious theories have been made to account for the 

 origin of the division-centres of the Metazoa. Biitschli ('91) was the 

 first to suggest that the micronucleus of Infusoria might be the pro- 

 tozoan analogue of the metazoan centrosome. Hertwig ('92) and 

 Heidenhain ('94) accepted the suggestion, and the latter, in partic- 

 ular, worked out a complicated theory of phylogeny upon it. The 



1 Loc. tit., p. 224. 



