REPRODUCTION 217 



to be a definite and permanent division zone which indicates the 

 future plane of division and which is not displaced even after diverse 

 mutilations of the body. Thus if Paramecium caudatum is cut 

 across either the anterior or the posterior end, the cell ordinarily 

 does not regenerate more than a ciliated surface on the truncated 

 end. It divides like a normal form, but the division plane is not 

 in the geometrical center of the mutilated cell, but in the geomet- 

 rical center of the cell as it was before the cutting (Fig. 108). The 

 same is true of Uronychia transfuga or U. setigera (Fig. 113). In 

 daughter cells of dividing Paramecium the future division zones 

 appear to be formed at an early period, and if a daughter cell is 

 cut in such a manner that the geometrical center is destroyed 

 without, however, destroying the nuclei, monsters of various types 

 are produced indicating a complete upset of the organization (Fig. 

 ION, f-o). In some cases, e. g., Frontonia leucas, the geometrical 

 center, or division zone, has a different physical appearance from 

 the remainder of the cell (Popoff, 1908, also mentioned by Hance, 

 1917, as occurring in Paramecium), but in the majority of cases 

 there is no morphological evidence of the plane of division during 

 inter-divisional stages. 



(a) Evidence of Nuclear Reorganization.— The two types of nuclei, 

 macronucleus and micronueleus, complicate the nuclear phenomena 

 at division. The macronucleus is more like a huge plastid of the 

 cell with active functions in metabolism, while the micronueleus is 

 generally interpreted as a germinal or racial nucleus, functioning 

 at division and particularly at conjugation. 



Reproduction of the macronucleus in the majority of ciliates is 

 analogous to that of a plastid. Division is direct with only a few 

 isolated cases showing evidences of spindle formation or of indefinite 

 chromosomes. In preparation for division, however, there is evi- 

 dence in many forms of profound changes in the make-up of the 

 nucleus destined to divide and some of these afford evidence of a 

 clear-cut reorganization of this important element of the ciliate 

 (see p. 93). 



In the less complicated types division of the macronucleus is 

 relatively simple. In Dileptus gigas, for example, the nuclear 

 material is in the form of many scattered chromatin and plastin 

 spheres, each of which divides prior to cell division (Fig. 46, p. 92). 

 There is no equal distribution of this chromatin to the daughter cells 

 but the daughter halves may go together to the daughter cell in 

 whose protoplasm they happen to lie. Some of the granules, how- 

 ever, those in the region of the division zone, may be represented in 

 each of the progeny. 



In forms with a single ellipsoidal macronucleus as in many of the 

 commoner types (e. g., Paramecium, Colpoda, Frontonia, Glaucoma, 

 etc.), the macronucleus simply elongates and constricts to form 



