Soon after the proliferation from the ectoderm has begun, the blind end of the digestive tract starts to push forward (Fig. 57, _D_). The cells in the most anterior part of its wall begin to divide, and the end grows in an anterior direction as a more or less solid rod. This rod extends, in some species, as far forward as the ectoderm, meeting the latter on the inner side of its antero-ventral surface. At this point an in-turning of ectodermal cells, in the form of a blind pit, develops, and later this pit, deepening to become a tube, forms the mouth cavity. Its inner end is from the beginning in contact with the anterior end of the digestive tract, or else it connects with the latter soon after its formation. The two flatten against each other, the cells draw away in the middle of the region of contact, and the cavity of the new mouth becomes continuous with the cavity of the old digestive tract. The mouth lies at first nearly terminal in position (Fig. 57, _E_), but by the forward growth of the body wall over and in front of the mouth to form the prostomium, the mouth comes later to lie more on the ventral surface. The short tube produced by the in-turned ectoderm forms only a short part of the digestive tract. It leads from the mouth opening to the new pharynx, and forms, therefore, only the buccal cavity. A similar ectodermal tube, the stomodæum, which develops in the egg-embryo, becomes not only the buccal chamber, but also the lining of the pharynx. The latter is, therefore, considered an ectodermal structure in the embryo. On the other hand, in the regenerated head the lining of the new pharynx arises from the anterior part of the endodermal digestive tract. We find, therefore, that the same organ, the pharynx, may arise in the same animal from distinct “germ-layers.” This result also has an important bearing on our ideas concerning the value and meaning of the so-called “germ-layers,” and has helped to bring about a revolution of current opinion as to the importance of these layers. The preceding account of the development of the head has shown that while certain of the new organs and layers arise from the same organs of the old part, yet this is not true for all of them. Thus while the ectoderm gives rise to ectoderm, the new muscles do not appear to come from the old ones, or even from other mesodermal tissues, but from the ectoderm. The old digestive tract gives rise to the greater part of the new one, but the new pharynx comes from the old endoderm, and not from the in-turned ectoderm. The nervous system does not arise from the old ventral cord, but from a proliferation of ectoderm. It has, thus, the same origin as the nervous system of the embryo. The origin of the new blood vessels has not been satisfactorily made out. The seta sacs arise from ectodermal pits as in the embryo.