THE INFUSORIA 



377 



(Fig. 23) and Kentrochona we also find a clear globule of achromatin 

 substance at the poles, but there are no true linin fibrils. In most 

 cases the meganucleus in fission or gemmation divides by simple 

 constriction into two approximately equal parts, but in Opalinopsis 

 (Fig. 19) and Anoplophrya (Fig. 30) it divides into a number of 

 unequal parts, and in Ephelota (Fig. 20), Podophrya, and others a 

 number of pieces are constricted off from it, each of which gives 

 rise to the meganucleus of a bud. 



The division of the micronucleus is always mitotic. The first 

 changes that are noticed are increase of size and the resolution of 

 the chromatic granule into a network of anastomosing fibrils. The 

 increase in size is usually considerable. In Colpidium, for example, 

 the diameter increases three- or four-fold (Hoyer). In Dendrocometes 

 the micronucleus increases from 2 p. to 10 /A in diameter. 







Fio. 24. 



Diagram to illustrate the structure and division of the nuclei of the Infusoria. 1, a micro- 

 nucleus in a state of rest, consisting of a spherule of chromatin in a clear vacuole ; 2, formation 

 of the spindle of linin fibrils, with a band of chromosomes on the equator ; 3, the chromatin 

 separated into two compact masses at the poles of the spindle ; 4, formation of the vacuole 

 round the chromatin, and dissipation of the spindle ; 5, a spherical meganucleus in a state 

 of rest ; 6, elongation of the meganucleus previous to division ; 7, constriction of the mega- 

 nucleus ; 8, division into two daughter meganuclei. 



After the increase of the micronucleus in size is completed, a 

 clearly-defined spindle of linin fibrils appears and the chromatin 

 network breaks up into a number of small chromosomes arranged 

 in an equatorial plane (Fig. 24). The chromosomes are so small 

 and their number is so great that they can neither be accurately 

 counted nor their method of fission determined. It seems probable, 

 however, that the two parties which travel towards the poles are 

 exactly equal in number. No satisfactory accounts have yet been 

 given, in the division of the micronucleus, of structures correspond- 

 ing with the centrosomes of the karyokinetic figures of other cells, 

 and in many cases that have been very carefully investigated 

 centrosomes are certainly absent. It is one of the most striking 

 features, perhaps, of the nuclear phenomena of the Heterokaryota 

 that centrosomes do not occur. The two parties of chromosomes 

 travel with considerable rapidity to the poles of the spindles, and 

 there, in many cases, they become compressed into a tiny compact 

 mass, leaving the spindle between them free from chromatin. The 



