Abstract
The coefficient of variation for the number of segments regenerated posteriorly by earthworms (Eisenia foetida) rarely exceeds 10 but the range may be over 100% of the mean. The manner of wound healing after transection (open or enteroparietal vs. closed or cicatricial) can not be responsible for the variation since under the conditions of these experiments all worms healed in the open manner. The angle of transection can be varied widely without any measurable effect on the number of segments regenerated. However, when the cut leaves a piece of ventral body wall and nerve cord extending posteriorly, the mortality is greatly increased and the number of segments formed by the survivors greatly reduced.
Three successive regenerations in the same individuals revealed statistically highly significant consistencies indicating a genetic factor behind variation in segment number. Data from wild populations in similar but widely separated sites and also the large number of segments regenerated by individuals with initially very large numbers of segments all support the view that these worms are genetically heterogeneous regarding segment number. However, the effect of these genetic factors cannot be great.
Bilateral asymmetry in number of segments regenerated indicates that the counting mechanism underlying variation can hardly be humoral because both blood and coelomic fluid are common to both sides of the body.