Nerve fibres push out from the end of the ventral nerve-cord into the new part, and groups of cells, often in process of division, appear in the old ganglia, even in those that lie a long distance from the anterior end. It is not improbable, Hescheler thinks, that new cells, as well as fibres, grow forward from the most anterior end of the nerve-cord into the new part. A mass of nerve cells and fibres appears in front of the old nerve-cord, and extends upwards and around the digestive tract, to meet over the anterior end of the latter in another mass of cells that have arisen from an early in-wandering of ectodermal cells. It is not improbable that the masses around the digestive tract (the commissures) and also the new ventral cord may also include cells that have had the same origin. A tubular invagination of ectoderm is formed at this time at the anterior end. It meets the anterior end of the digestive tract; the two fuse, and the communication of the digestive tract with the outside is established. The pharynx develops from the anterior part of the digestive tract, which after Hescheler’s operation may contain some of the original ectodermal stomodæum, since only five of the anterior segments were cut off, and the embryonic stomodæum extends somewhat behind this region. In another experiment, carried out by Kroeber, somewhat more of the anterior end was removed, but the result was the same (Fig. 59), so that it is clear that the new pharynx may be formed from the old endoderm. [Illustration: FIG. 59.--After Kroeber. Regeneration of anterior end of _Allolobophora fœtida_, after removal of six segments. The first stomodæal invagination had been destroyed. The new pharynx is developing from the endoderm.] Hescheler leaves several points still unsettled, more especially the origin of the cells that give rise to the new musculature, but it is almost impossible to make out their origin in this animal, owing to the presence of the lymph cells. Hescheler’s discovery that the cells of the lymph plug do not themselves, in all probability, contribute to the new part, is an important result, and shows that these seemingly undifferentiated cells do not possess the power of giving rise to the different kinds of new tissues. The in-wandering of cells into this solid plug from the ectoderm, and perhaps also from other sources, and their subsequent union to produce the definitive organs, is also a point of capital importance, especially as it puts us on our guard against a too ready acceptation of the view that all cells in a mass that have the same general and undifferentiated appearance have had a similar origin, and in showing that apparently indifferent cells may really carry with them into the new part those characters that determine their fate. Other cells, apparently equally undifferentiated, and lying in the same position, may have quite different possibilities.