From his studies on the toxicity of Pb ions in cell division in onion root tips, which he explained on the basis of the binding and eliminating of a SH-group, and from his experiments designed to show the effect of SH-groups on rate of growth depending on cell multiplication, as in growing onion root tips, Paramecia cultures, and healing of wounds, Hammett1 postulates his theory that the SH is the “mitotic hormone” and the wound hormone or essential chemical factor in cell proliferation following trauma.With the thought that a specific mitotic hormone would at least accelerate, if not also actually increase, regeneration since the latter depends on cell division and proliferation in its earlier stages, we performed a series of experiments on regeneration of the polychaete worm, Podarke obscura. This animal was chosen because one of us (S. M.) has had extensive experience with this material and had previously made a long study of its regenerative process.2The procedure was simple. Under a magnifying glass ...