Two Papers on the Finer Structure of Nerve Cells

The ventral nerve cord grows forward into the above mass as nerve fibers accompanied, probably, by some cells from the old cord. It is interesting to note here that the entire cord, at least as far back as the fifteenth segment, shows most active mitotic divisions of ganglion as well as of other cells. To this forward growth from the old cord is added a collection of many cells that migrate in, separately, from the new epidermis that grew over the cicatrix. These cells furnish the main part of the new brain. The new epidermis over the cicatrix grows backward as a small funnel, which meets the old intestine as it elongates into the new tissue. The ingrowth ultimately opens into the old intestine and is thought probably to form the digestive tract in the new head as far back as the fourth segment, where the new pharynx will be formed from the old intestine. The making of a new head in the earthworm thus involves elongation of old organs, transformation of some of them, and in the case of the nervous system marked change of activity even in parts remote from the wound; in addition there is a large element of new formation from cells of an embryonic and undifferentiated character. E. A. A.