Posterior regeneration of normalNereis diversicolor O. F. Müller includes two stages: regeneration of the pygidium, regeneration of setal segments. Regeneration of the pygidium results from mitotic proliferation of epidermal cells in close proximity to the wound after junction between epidermis and intestinal epithelium at the section plane; correlatively, mesodermal cells accumulate behind the healing area. Pygidial differentiation is precocious and results both from a rearrangement of mesodermal cells inside the blastema and the differentiation of epidermal and mesodermal cells in the area of the future pygidium. Regeneration of setal segments is caused by the association of two different levels in a gradient of antero-posterior differentiation (regenerated pygidium-last segment of the stump); it results from the activity of a narrow undifferentiated area (proliferation zone) which persists just in front of the regenerated pygidium. 2. Externally, posterior regeneration of decerebratedN. diversicolor O. F. Müller consists only of the differentiation of the pygidium. However, an histological study of these regenerates shows a beginning of mesodermal segmentation and parapodial differentiation in the area between the regenerated pygidium and the stump. These observations demonstrate that: the brain plays no role in the activation and division of the blastema cells, at least during the first steps of the regeneration process; the activity of the nervous system, mainly the nerve cord, is not interfered by extirpation of the brain; the differentiation of the pygidium escapes from the control exerted by the brain; the metamerisation of the mesoderm is independent of the growth and segmentation of the regenerated epidermal wall; the absence of regenerated setal segments results probably from the morphogenetic inactivity of the epidermis of the prepygidial area.
