376 THE INFUSORIA 



plasm in some species. It is, however, always extremely thin and 

 flexible. 



THE MICRONUCLEUS. Whilst it cannot yet be said that a 

 micronucleus has been proved to exist in all Ciliata and Acinetaria, 

 the careful researches of recent years renders it extremely probable 

 that one or more micronuclei form an essential feature of their 

 organisation. The difficulty of determining this important fact 

 with certainty is that, during the long period of rest which the 

 micronuclei pass through between the acts of fission, gemmation, or 

 conjugation, the chromatin, as well as the achromatin, elements 

 shrink into such a small compass that they are not easily seen. 

 The usual appearance of a micronucleus in the resting-stage is that 

 of a minute irregular granule lying in the centre of a perfectly clear 

 vacuole. No lines nor dots can be seen in the vacuole, but it seems 

 probable from events which occur during division that the clear 

 substance contained in the vacuole is the same as the achromatin 

 of the karyokinetic figure. 



The size of the micronucleus in the resting-stage is rarely more 

 than 1 p,, but more frequently it is 1 /x, or even less in diameter. It 

 is 2 p. in Dendrocometes, 4-5 p in Prorodon, and 12-14 //, in Para- 

 moecium bursaria, in which species it seems to reach its maximum 

 size. 



The staining reactions of the micronucleus in rest are the same 

 as those of the chromatin of the meganucleus. 



The number of micronuclei in the individual varies considerably. 

 In most species there is only one (Paramoecium caudatum, Colpidium 

 colpoda, Vorticella monilata, etc.) ; in others there are normally two 

 (Paramoecium aurelia), or three (Spirochona), or any number up to as 

 many as twenty-eight (Stentor roeselii). The number is not always 

 constant in the same species. Maupas, for example, found three 

 micronuclei in some individuals of Paramoecium aurelia. In Den- 

 drocometes the number varies from two to five ; Spirochona gemmipara 

 has, according to Hertwig, normally three micronuclei; but in several 

 specimens examined in Manchester, Hickson could only find one. 



DIVISION OF THE NUCLEI. There is a very important difference 

 to be observed in the division of the meganuclei and micronuclei. 

 The meganuclei divide only during fission or gemmation, except in 

 those cases mentioned above in which the meganucleus breaks up 

 into a few or many pieces, after fission of the individuals. In all 

 cases the division is strictly amitotic. In some cases a concentra- 

 tion of the chromatin occurs along lines parallel with the longer 

 axis of the meganucleus, giving the appearance of longitudinal 

 striations or thin delicate chromosomes with anything but the higher 

 magnifying powers. In some cases (Spirochona) these lines are 

 crowded together at' the narrowest part of the constriction, and 

 may have the appearance of a broad equatorial plate. In Spirochona 



