The cell in development and inheritance . sider especially the achromatic figure. The basisof our knowledge in this field was laid by Richard Hertwig through hisstudies on an infusorian, SpiiocJiona (JJ^, and a rhizopod, Actmo-sphceritmi (84). In both these forms a typical spindle and equatorialplate are formed mside the nuclear membrane by a direct transfor-mation of the nuclear substance. In Spirochona (Fig. 38, A-C^ a DETAILS OF MITOSIS 91 hemispherical end-plate or pole-plate is situated at either poleof the spindle, and Hertwigs observations indicated, though theydid not prove, that these
The cell in development and inheritance . sider especially the achromatic figure. The basisof our knowledge in this field was laid by Richard Hertwig through hisstudies on an infusorian, SpiiocJiona (JJ^, and a rhizopod, Actmo-sphceritmi (84). In both these forms a typical spindle and equatorialplate are formed mside the nuclear membrane by a direct transfor-mation of the nuclear substance. In Spirochona (Fig. 38, A-C^ a DETAILS OF MITOSIS 91 hemispherical end-plate or pole-plate is situated at either poleof the spindle, and Hertwigs observations indicated, though theydid not prove, that these plates arose by the division of a large nucleolus. Nearly similar pole-plates were somewhat described bySchewiakoff (88) in Eiiglypha (Fig. 39), and it seems clear that theyare the analogues of the centrosomes or attraction-spheres in higherforms. In EtLglena, as shown by Keuten, the pole-plates, or theiranalogues, certainly arise by division of a distinct and persistent intra-nuclear body (nucleolus or nucleolo-centrosome ) which elon-. Fig. 40. — Mitosis in the flagellate, Eiiglcna. [KelTEX.]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 intranuclearspindle. C. Later stage. D. The nuclear division completed. gates to form a kind of central spindle around which the chromatinelements are grouped (Fig. 40); and Schaudinn (95) described asimilar process in Aviwba. Richard Hertwigs latest work onInfusoria (95) indicates that a similar process occurs in the micro-nuclei of Parantceciuni, 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 inCcratijini (Lauterborn, 95), and as in the Infusoria the entirenucleus transforms itself into a fibrillar spindle-like body. 92 CELL-DIVISION Still si
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