Relationships between Feeding Organelles and Encystment in Oxytricha fallax Stein

SYNOPSIS. In Oxytricha fallax , extrusion of all the excretory crystals and some refractile bodies occurs when food is with held. The precystic animals thus resulting become transparent and undergo dedifferentiation of organelles. First the undulating membrane and cytostome are entirely resorbed, producing astomial forms. Then the adoral membranelles vanish gradually, accompanying which the somatic ciliature such as the cirri comes to be resorbed. Occasionally, early precystic animals retaining the intact feeding organelles reorganize spontaneously (physiological regeneration), but they become irreversibly encystable by high‐temperature treatment; for hastening resorption of feeding organelles accelerates encystment. When transparent animals at various precystic stages are cut in half the posterior fragment (PF) always encysts faster than the anterior one (AF) from the same parent, except that some of the AFs from early stages reorganize into normal vegetative forms. The time difference for completion of encystment between an AF and PF pair is at early stages larger than at later ones. If the anterior part of AF is further excised, the time difference for encystment between AF and PF from the same source becomes smaller or indistinguishable, with the exception that some AFs from early stages regenerate instead. Further, when the anterior part of whole precystic animals is removed, the time for their encystment is reduced. Also in this case, the fragments from early stages normally regenerate. From these results, it is presumed that the adoral membranelles tend to inhibit the encystment process as they do the initiation of an oral primordium in dividers and regenerants of Stentor and other ciliates. Accordingly, when resorption of membranelles becomes extensive as precystic processes progress, their dominance over and inhibition of cyst formation may be diminishing. From the fact that regeneration of fragments from early stages into normal forms may be due to retention of the undulating membrane, its resorption is considered to be an essential prerequisite for the initiation of irreversible encystment.