THE OPALINID CILIATE INFUSOKIANS, 11 



Well forward in the axis of the body are usually two nuclei 

 (always two, except in transient conditions associated with fission, 

 and except in the presexual and sexual period). In this species the 

 nuclei are usually somewhat pear-shaped and their more pointed 

 ends, directed toward each other, are, in a majority of individuals, 

 connected by a delicate thread which is merely an attenuated thread 

 of nuclear membrane, remaining from a preceding nuclear division 

 not as yet quite carried to the point of complete separation of the 

 two daughter nuclei. This connecting thread is very tough. In a 

 number of instances I have had under observation living individuals 

 which had been broken in such a way that one of the two nuclei 

 was firmly held within one body fragment while the other nucleus 

 lay outside in the water, still connected, however, by the thread 

 yviih its sister nucleus. Intermittent pressure upon the cover glass, 

 under these conditions, may set up violent currents in the liquid, 

 causing the external nucleus to whip about violently, but in no such 

 case have I succeeded in breaking or even stretching the thread. It 

 is strong and inelastic. From this, one can infer that the nuclear 

 membrane as a whole is tough and inelastic. 



Fission is frequent in Protoopalhm intestinalis, as it is in other 

 Opalinidae. We therefore find the nuclei in different conditions in 

 different individuals, according to the mitotic stage in which we 

 observe them. As the condition of the nuclei in the several species 

 is emphasized in the taxonomic portion of this paper, and as the 

 series of these conditions is of great significance, it is important 

 to review in detail the mitosis in this introductory description. JX 

 is convenient first to describe the reticulate nucleus, corresponding 

 to the so-called " resting nucleus " ^° and then to describe mitosis. 



The reticulate nucleus (fig. 4, a). — The caryotheca is of appreciable 

 thickness and is tough and inelastic. Due probably to the tension 

 of its membrane, the nucleus may often be ellipsoidal or more or less 

 pointed at one end or at both ends. But in the reticulate stage the 

 nuclei are sometimes approximately spheHcal. The nuclear mem- 

 brane is persistent in all phases of the nuclear behavior, not disap- 

 pearing even during mitosis. Within the caryotheca there is, of 

 course, both achromatic and chromatic material. The former shows 

 numerous granules and fibrillae or films. The chromatin is in the 

 form of (1) large flat chromatin masses interconnected by branching 

 chromatin threads, and (2) granules somewhat larger than the achro- 

 matic granules, and themselves connected by chromatin threads. 

 The reticulate nucleus gives the appearance of an abundant reticulum, 

 both chromatic and achromatic. The large chromatin masses and 



'"This is not tlie usual condition of the nuclei in Protoopalina intcstinalis. The nuclei, 

 when not actively engaged in division, are somewhat elongated and arc really in a late 

 telophase condition. 



