SPECIAL MORPHOLOGY OF THE PROTOZOAN NUCLEUS 265 



matic" substance, which are formed from the linin reticulum (■Hertwig) 

 and upon which the chromatin granules are strung. At a " certain 

 period in mitosis " (Brauer), a period which Hertwig identified as 

 " maturation mitosis," minute bodies, analogous to centrosomes, 

 emerge from the nucleus and take a position in this extra-nuclear 

 mass of achromatic material (Fig. 140, F). At this stage of division, 

 therefore, there are structures which, more than anything else in the 

 Protozoa, resemble the astral system in the cytoplasm of many meta- 

 zoan cells. They occur, however, only at certain periods and are 

 not characteristic of ordinary vegetative mitosis. 



Nuclear division in Euglypha as described by Schewiakoff ('88) is 

 characterized by stages which are remarkably similar to those in 

 Metazoa. The chromatin passes through a spireme stage and breaks 

 into chromosomes by transverse division of this spireme ; a spindle is 

 formed from "achromatic material," and Polkorperchen, which are 

 strikingly suggestive of centrosomes, form the poles of the division- 

 figure (Fig. 23, p. 55). The origin, again, of the spindle-fibres and of 

 the pole-bodies was not determined. Schewiakoff assumed that the 

 latter are derived from the cytoplasm. 



In all of these cases, with the exception of certain maturation 

 phases of Actinosphcerium, the entire division-figure remains inside of 

 the nuclear membrane. The substance of the spindle, therefore, must 

 arise from within the nucleus, and although it has not been definitely 

 proved, there is good reason to believe that this substance is con- 

 tained, during the resting phases, in the so-called " nucleolus " or 

 intra-nuclear sphere (division-centre). There is no doubt upon this 

 point in regard to the division-figure of the simple flagellates 

 (Euglena, etc.), for the clearly defined achromatic body can be traced 

 throughout all stages. The connecting strand in Englena is not 

 fibrillated, and therefore a true spindle is lacking, but there seems 

 little room to question the analogy between such a strand and the 

 fibres in forms like Actinosphcerium, Spirochona, etc. Indeed, the 

 relation of the single strand to the fibrillated spindle is apparently 

 well marked by an intermediate form in Paramcecium (Hertwig, '96), 

 where the central portion of the division-figure is a single strand 

 which widens and becomes fibrillated at the ends (cf. Fig. 

 139. H). 



2. Extra-nuclear Division-centres. 



We now pass to a consideration of more complicated types of 

 protozoan nuclei, in which, during the resting phases, the kinetic 

 structures are outside of the nuclear membrane. 



The Centralkorn in Heliozoa, with its radiating fibres which form 

 the axial filaments of the pseudopodia (cf. p. 82), have often, and 



