DETAILS OF MITOSIS 



91 



hemispherical " end-plate " or "pole-plate" is situated at either pole 

 of the spindle, and Hertwig's observations indicated, though they 

 did not prove, that these plates arose by the division of a large 

 "nucleolus." Nearly similar pole-plates were somewhat described by 

 Schewiakoff ('88) in Englypha (Fig. 39), and it seems clear that they 

 are the analogues of the centrosomes or attraction-spheres in higher 

 forms. In Ejiglena, as shown by Keuten, the pole-plates, or their 

 analogues, certainly arise by division of a distinct and persistent intra- 

 nuclear body ("nucleolus" or " nucleolo-centrosome ") which elon- 



Fig. 40. — Mitosis in the flagellate, Euglena. [Keuten.] 



A. Preparing for division ; the nucleus contains a " nucleolus " or nucleolo-centrosome sur- 

 rounded by a group of chromosomes. B. Division of the "nucleolus" to form an intranucleai 

 spindle. C. Later stage. D. The nuclear division completed. 



gates to form a kind of central spindle around which the chromatin 

 elements are grouped (Fig. 40); and Schaudinn ('95) described a 

 similar process in Aviceba. Richard Hertwig's latest work on 

 Infusoria ('95) indicates that a similar process occurs in the micro- 

 nuclei of Paramcecinni, which at first contain a large " nucleolus " 

 and afterward a conspicuous pole-plate at either end of the spindle 

 (Fig. 38, D-H). The origin of the pole-plates was not, however, 

 positively determined. A corresponding dividing body is found in 

 Ceratiiivi (Lauterborn, '95), and as in the Infusoria the entire 

 nucleus transforms itself into a fibrillar spindle-like body. 



