These video sequences were recorded (and displayed) in real time, using Nomarski (DIC) optics. The full frame width for each is 56 um. Stage IV cells are elongate, with a reduced ridge, very active anterior flagellum, and limited attachment to the substratum. They swim rapidly through the fluid medium, spiraling about their long axis, following a straight, or slightly-corkscrewed, path.

STAGE IV (videosequences G,H): The last developmental stage recognized here consists of freely-swimming flagellate cells that are reaching their maximum lengths. Stage IV cells have length to width ratios that are greater than two, with lengths ranging from 14 to 20 um. The anterior process and nucleus therein are both quite tapered. The cells often retain some curvature, but tend to swim in straight lines, usually rotating around their long axes. Frequently, they make contact with the substratum, where they appear transiently attached. Often, attached cells "revert" to behavior and morphology more characteristic of stage III cells or they translocate in a gliding manner with the anterior flagellum oscillating slowly. Such movements are accompanied by bulk cytoplasmic flow, reminiscent of fountain streaming, and euglenoid deformations of the entire cell body.

Cells at stage IV increase in number as those at stage III decline. Then, late in the transformation, an increasing number of cells appear to be at stage III of development once again. However, rather than elongating, they are in the process of rounding up into the cyst-like, immobile cells which eventually replace swimming flagellates in transformation buffer.

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