218 BIOLOGY OF THE PROTOZOA 



With but few exceptions the division plane is through the center of 

 the l)o(Iy and in a plane at right angles to the long axis of the cell. 

 The externals of division are similar to division in other groups, 

 with preliminary division of the plastids and nuclei and final 

 division of the cell body. As in flagellates and some rhizopods the 

 cup or test-dwelling forms divide within the parent cup, one of the 

 daughter individuals migrating and forming a cup for itself. In 

 some forms the daughter individuals remain and share the old 

 house {{Cothurnia imjenita). 



Where a tightly-fitting cell-covering is present as in Coleps hirtus, 

 it is divided transversely and the missing parts are regenerated by 

 the daughter organisms (Fig. 65, A, B,C, p. 128). In some Infusoria 

 as in the other groups, division in many cases is incomplete, the 

 daughter individuals remaining attached end to end as in Polysjnra 

 delagei or llaptophrya gif/antea (see chain building in Ceratknn indtur, 

 Kofoid), Or daughter indi\iduals may remain attached by incom- 

 plete division of their stalks, thus giving rise to arboroid colonies 

 of different types (Vorticellida? mainly). 



In some forms, probably in the majority of ciliates, there appears 

 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 the division plane, however, is 

 not in the geometrical center of the mutilated cell, but in the 

 geometrical center of the cell as it was before the cutting (Fig. 

 103). The same is true of UronycJiia transjuga or JJ . setigera 

 (Fig. 108). In daughter cells of dividing Paramecium the future 

 di^'ision 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. 103, 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 men- 

 tioned by Hance, 1917 as occurring in Paramecium), but in the 

 majority of cases there is no morphological evidence of the plane 

 of division during resting stages. 



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

 macronucleus and micronucleus, complicate the nuclear phenomena 

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

 cell with active functions in metabolism, while the micronucleus 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 



