I shall describe the principal changes that have been found to take place. When the anterior end of lumbriculus or of tubifex is cut off, the cut-surface very quickly closes, as a result of the contraction of the body wall. According to some investigators, the circular muscles are chiefly concerned in the closing, but according to others the longitudinal muscles bring about the result. The cut-end of the digestive tract is pulled a little inward, and its end also closes (Fig. 57, _A_). For a day or two no important changes can be observed to take place, but new ectoderm soon appears over the cut-surface. This ectoderm arises in all cases from the old ectoderm, and as it increases in amount the old ectoderm is pushed back from over the cut-end, leaving a layer composed of a single row of cells over the end. Since nuclei in process of division are rarely present before these initial processes begin, it is probable that the changes are due, in large part, to an out-wandering of ectodermal cells, or, what amounts to the same thing, to the leaving behind of cells as the old ectoderm withdraws from the cut-end. In the new ectoderm over the end, an active process of proliferation takes place (Fig. 57, _B_), that leads to the production of a large number of cells lying within the new part. The ectoderm has at this time begun to bulge outward, so that the proliferated cells come to lie within the dome-shaped beginning of the new head. There appears to be some difference in the number and in the location of the proliferations in different species. In general, the new cells arise from the ventral and ventro-anterior region of the dome-shaped ectodermal covering of the new part. Most of this new material gives rise to the brain, commissures, and ventral nerve-cord (Fig. 57, _C_). The cells giving rise to these structures in tubifex come from two ventral regions of proliferation that extend along the sides and dorsally to the anterior end in front of the digestive tract. Where the two masses meet above and in front, the brain is formed.[92] The cells that do not take part in the formation of the nervous system give rise to the muscles and connective tissue of the new head. These cells lie especially at the outer sides of the proliferated mass. The origin of the new muscles from ectoderm stands in sharp contrast to the current ideas in regard to the origin of new tissues, and yet it is a point on which the more recent investigators are entirely in accord. Michel, Hepke, and von Wagner have arrived at the same conclusion after a careful examination, and there seems to be no reason for refusing to accept their results. The theoretical importance of this discovery will be discussed later.