1. A serial-sectioning technique is described for cysts of the potato root eelworm. The distribution of hatching among eggs contained in the cysts is investigated by this technique.
2. It is shown that there are significant differences in the numbers of larvae emerging from different parts of the cyst, i.e. hatching is not random. There is a tendency for eggs near the cyst wall to hatch sooner than eggs nearer the centre of the egg mass.
3. The position of eggs in relation to the natural openings of the cyst, at the neck and vulva, is apparently without influence on the distribution of hatching.
4. Although the cyst wall is permeable to the hatching stimulant it is argued that the hatching pattern is more likely to be due to a gradient of oxygen tension within the cyst.