An experimental and morphological analysis of the tail bud mesenchyme of the chick embryo

In the chick embryo, the tail bud reaches its maximum length at about stage 22 of Hamburger and Hamilton, after which it starts to regress. By this stage the neural tube and notochord extend right to the tip of the tail, but the somites do not do so, the terminal tail bud mesoderm never becoming segmented. The investigation is concerned with analysing why this mesoderm fails to segment. When tail buds were explanted to the chorio-allantoic membrane, they continued to form somites only until the "correct" number had segmented, i.e., the tail bud formed no more somites when isolated from the embryo than it would have formed if undisturbed. Morphological studies suggest that in the normal embryo massive cell death overtakes the tail bud mesoderm before it can segment. It is suggested therefore that cell death may be a contributory factor in preventing segmentation.