Another illustration may be given from certain experiments in the regeneration of _Planaria lugubris_. If the posterior end is cut off just in front of the genital pore, as indicated in Fig. 41, new material develops at the anterior cut-edge, and in a few days a new head is formed out of this new material. A new pharynx appears in the new tissue immediately in front of the old part. It lies, therefore, just behind the new head. The proportions of the new worm are at this time very different from those of a typical worm, since the head is much too near to the new pharynx and to the old genital pore. New material is now produced in the region behind the head and in front of the pharynx, so that the head is carried further forward until the new worm has fully assumed the characteristic proportions. As the new head is formed the old part loses its material, so that it becomes flatter and narrower, and if the worm is not fed the old part may lose also something of its former length. If the worm is fed, however, as soon as the pharynx develops the old part loses less and the new part grows forward more rapidly. The most striking phenomenon in the growth of the new worm is the formation of new material in the region behind the head. The result of this growth is to carry the head forward and produce the characteristic form of the animal. This change is all the more interesting since the growth does not take place at a free end, but in the middle of the new material. It is only by the formation of new material in this region that the head is carried to its proportionate distance from the pharynx. It appears that in some way the growth is regulated by influences that determine the form of the new organism. [Illustration: FIG. 41.--Posterior end of _Planaria lugubris_, cut off between pharyngeal and genital pores. Figure to left shows the piece after removal. The four figures to the right show the regeneration of the same piece, drawn to scale. As soon as the new pharynx had developed, the worm was fed. The experiment extended from November 17 to January 8.]