Differentiation During Encystment and Excystment in Oxytricha fallax

SYNOPSIS During encystment of Oxytricha fallax, a wall composed of 4 distinct layers, each derived from a different kind of endoplasmic vesicle, is formed between the 2 unit membranes that cover the vegetative cell. Numerous autophagic vacuoles arise in the endoplasm and later (during excystment undergo internal changes comparable to those characteristic of food vacuoles. Mitochondria aggregate into a band. The 2 macronuclei fuse and their nucleoli become homogeneous. Except for the 2 cell membranes, all visible cortical structures, including cilia, kinetosomes, and microtubules, disappear. Despite the absence of visible ciliature in the mature cyst, the various primordia of the normal vegetative ciliature arise during excystment in the same positional relations to one another as is characteristic of developments during cell division.