Differentiation in Protozoa*

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Abstract
SYNOPSIS. Two main kinds of morphogenetic events occur among protozoa: (1) reproduction; (2) processes of reorganization, including encystment, excystment, sexual processes, structural and physiological adaptations exhibited in the course of complex life cycles, and restorative events elicited by environmental stimuli. Examples of such events are discussed with reference both to their own intrinsic interest and to their value in the study of cellular differentiation in general. Stomatogenesis in ciliates provides a remarkable instance of extensive and rapid differentiation. The phenomenon may be studied to advantage not only in reproduction but also in the macrostome‐microstome transformation. Other simple transformations in protozoa providing unparalleled material for experiments on cytodifferentiation under controlled conditions are excystment and encystment, the amebo‐flagellate transformation, and the leishmania‐leptomonad change. The more complex transformations occurring in the life cycles of trypanosomes and in the appearance of sex under the influence of an insect hormone provide additional material of basic significance to an understanding of differentiation.