CONJUGATION, DIVISION, AND ENCYSTMENT IN PLEUROTRICHA LANCEOLATA

Abstract

1. Pleurotricha lanceolata is a hypotrichous ciliate belonging to the family Pleurotrichidæ. The species has as its chief characteristic six anal cirri, divided into two groups. The anterior of the two groups consists of four cirri—three very large ones arranged in an oblique row, and a smaller one, a little forward of the posterior end of the row. The second group includes two cirri, both of them very long and projecting well beyond the posterior end of the animal. The usual size of vegetative individuals is 140 µ. 2. The process of division does not differ particularly from that in other infusoria, except that in the very early stages an endosome appears and divides, the products remaining connected by an intradesmose. This has been described in Paramœcium trichium but in few if any other species. The process of division is initiated by the appearance of a rudimentary adoral zone and a kernspalt in each of the macronuclei. The micronuclei divide by typical mesomitosis. When division of the latter is virtually complete the macronuclei follow suit, dividing amitotically, and then the cell itself divides. All organelles appear to be regenerated, the old ones being absorbed. 3. Encystment may occur at any time, and appears to bear no relation to periods of depression, to division or to conjugation. The old macronuclei are extruded bodily from the cell, and a single micronucleus remains. It is uncertain whether the other is always extruded with the macronuclei or fuses with the first micronucleus. An uncertain number of micronuclei are formed from the single remaining micronucleus, and from these the normal nuclear complex is rebuilt, the process being complete at the time the animal is ready to leave the cyst. 4. The nuclear changes which occur during conjugation are essentially the same as those described for other ciliates. There are three maturation divisions, an interchange of pronuclei, and two or rarely three cleavage divisions. The four products of the second division are at first alike, but one soon enlarges and eventually gives rise to the new macronuclei of the reorganized exconjugant. One of the others degenerates and the other two form the new micronuclei. The old macronuclei degenerate after separation of the exconjugants. There may or may not be one division of the latter before reorganization is completed. 5. Reduction occurs in the second maturation division. The diploid number of chromosomes is forty, as nearly as can be determined, and the haploid number twenty. The chromosomes are dumbbell-shaped in the first maturation division, and rod-shaped in the second and third. They are also rod-shaped in the cleavage divisions, but the shape then is unlike that in the maturation divisions or in vegetative division. 6. Each of the divisions in conjugation differs from all the rest and from vegetative division. 7. Conjugation occurs but rarely in the race of Pleurotricha used, and under the conditions of culture virtually always results in the death of the conjugants a few days after separation. In only one instance out of a hundred did it result in anything like rejuvenation, and even in this case the daughter race died within a month. 8. Although the details of conjugation are much alike in Oxytricha fallax and Pleurotricha lanceolata there is no obvious relation between the number of chromosomes of these two very similar species.