1. The anlage of the pharynx is formed at one extremity of the yolk-cavity of the embryo, that of the head at the other.2. Up to the period of the formation of the pharynx the yolkcavity is surrounded by a layer of parenchymal cells and these by a thin epithelial sheet. The parenchymal cells multiply by direct and by indirect division and become reduced in size as they increase in number.3. The ectodermal cells begin to multiply rapidly by direct division after the pharynx has been formed, and thus give rise to a columnar epithelium. The dorsal cells at no time develop cilia, while the vential cells produce them before becoming columnar in form.4. Tissue differentiation begins in the parenchyma shortly before the pharyngeal anlage appears. From the parenchyma are developed intestinal cells, muscle cells, nerve cells, rhabdite cells and gland cells, as well as branched connective-tissue cells and possibly leucocytes. Certain of the parenchyma cells seem to multiply without undergoing further differentiation. Intestinal, nerve and muscle cells multiply by direct division, while the parenchyma cells multiply mainly by indirect division.5. The central nervous system is first developed in the region of the head and from there the lateral cords grow posteriorly.6. When removed from the cocoon very young embryos soon die. Slightly older ones die if injured. After the pharynx has become functional the embryos may be cut into two or more parts which will live. Pieces from which the head has been removed will not develop a new head unless they contain well-developed nerve cords.7. In regeneration the new ectodermal cells arise from the old by direct division. New nerve cells, intestinal cells and muscle cells seem likewise to arise from preëxisting cells by direct division. It is possible, however, that they may spring from the large cells of embryonic type situated in the parenchyma. From the latter the other tissues mainly arise.8. The new pharynx arises immediately posterior and ventral to the region into which the intestinal contents are forced by general muscular contraction and in response to stimuli arising from this region.9. A new head is regenerated only in tissue produced at a cut surface, and with definite axial relations to the surface. The direct stimulus to head formation arises from an exposed chief coördinating region of the central nervous system. The head is formed with radial symmetry about the tip of the main intestinal branch extending to the cut surface in the vicinity of the exposed nervous system.10. After head and pharynx have been differentiated the activities tending to restore the piece to a worm of normal form and proportions become especially marked.11. During regeneration highly differentiated tissues are destroyed unless they may be directly utilized in the formation of new parts.