SPECIAL MORPHOLOGY OF THE PROTOZOAN NUCLEUS 259 



it be a portion of the karyosome, which, in several Sporozoa, has been 

 shown to contain two differently staining substances. 1 



In still other Sporozoa there are distinct spindle-fibres, which in 

 some cases (e.g. Monocystis agilis, Wolters, '91) consist of two sets, 

 and their presence in resting nuclei cannot be made out. In all Spo- 

 rozoa, however, the origin of the spindle-substance and the division- 

 centre is difficult to make out because of the disappearance of the 

 nuclear membrane during division. In the case cited by Siedlecki, 

 however, there appears to be no doubt that the division-centres, with 

 their spindle-fibres, arise within the nucleus. 



In other forms of Protozoa there is usually some evidence of the 

 division-centre in the resting cell as well as in the mitotic figure. 

 The substance of such centres differs from the chromatin of 

 the nucleus in its different staining reactions, and from the sub- 

 stances in the cytoplasm by its more intense coloration. In some 

 cases, such centres are found within the nuclear' membrane dur- 

 ing all phases of cell-life (the majority of Mastigophora and Infusoria 

 and many Rhizopoda). In other cases they are permanently outside 

 of the nuclear membrane (Noctiluca, Paramceba, Heterophrys, Sphce- 

 rastruni}. Again, they may be intra-nuclear during some phases and 

 extra-nuclear during others (Tetramitus, Actinosphcerium, Acantho- 

 cystis\ 



I. Intra-nuclear Division-centres. 



The least-differentiated division-centres are those which are per- 

 manently within the nucleus. They are found in all classes of the 

 Protozoa, and have been variously interpreted. For a long period 

 they were erroneously regarded as nucleoli, but since their true func- 

 tion was first suggested by Keuten ('95) they have been variously 

 regarded as "nucleolus-centrosomes " (Keuten), centrosomes (Hert- 

 wig, '96), and spheres, equivalent to centrosome plus attraction- 

 sphere (Calkins, '98). 



In the majority of the Flagellidia the division-centre is clearly 

 defined and distinct from all other parts of the cell. Its changes in 

 form, which were first made out in Euglena by Blochmann ('94) and 

 by Keuten ('95), can be easily followed in almost any species of Euglena. 

 It lies in the centre of a spherical group of chromatin granules which 

 are connected with one another by a linin reticulum, the whole being 

 inclosed within a firm nuclear membrane (Fig. 136). During resting 

 stages of the cell it is globular or ellipsoidal in form, but during 



1 The " centrosomes " which Labbe ('96) described in Klossia eberthi, Bananella lacazei, 

 and Pfeifferia gigantea must be regarded with doubt until their relation to the nucleus 

 in division is made out. 



