At the beginning of each film, the entire visual field is covered with bacteria except near the left end, where slime mould cells are intermingled with the bacteria upon which they feed.

As the slime mould cells devour the bacteria and rapidly increase in number, they gradually extend their territory towards the right.

In the regions where the bacteria have been eaten up, some of the slime mould cells start to send out chemotactic signals periodically. The signal substance is sensed by the surrounding cells, which in turn rapidly make and secrete the same substance. They also start to move towards the direction the original signal came from.

In this way, periodical chemotactic signals propagate outwards, while the cells move inwards. The opposite directions of cell movement and wave propagation are clearly seen in the high time-resolution movie (see Movie 2 shown below). The cells gradually gather and pile up to form cell aggregates, which then elongate to form motile structures called "pseudoplasmodia" or more commonly "slugs". The anterior end of the slug is called the "tip".

In the two species shown here, each slug leaves a thin string-like thing trailing behind. This is the "stalk" which is made up with cells each filled with a large valuole and covered with a rigid cell wall, almost like plant cells. The stalk goes all the way up to the tip, where the cells in contact with the end of the stalk rapidly develop large vacuoles and cell walls to become mature stalk cells.

In D. discoideum, the stalk is not formed until the slug starts to transform into a fruiting body. So, slugs of that species move in a manner much more like real slugs.

Why are the waves of chemotactic signals visible?

The dark and light stripes seen to be moving in the films reflect the shape changes of the cells. When a cell receives a signal, it sort of shrinks first, then extends protrusions ("pseudopods") towards the signal source and becomes elongated in that direction. It continues to proceed for a short period (a few minutes) towards the signal source. It then stops and resumes the non-elongated shape until the next wave comes. Because all the cells along the wavefront do this synchronously, even the slight changes in the scattering of light caused by minute cells result in visible, macroscopic patterns.

The chemotactic substance is known to be cyclic AMP for D. rosarium shown here and some other species of the genus Dictyostelium such as D. discoideum. Species of the genus Polysphondylium uses a modified dipeptide called glorin as its chemoattractant during the aggregation process.