These results show very clearly that in some way the development of the typical form of the tail influences the rate of growth at different points. The more rapid growth takes place in those regions at which the lobes of the tail are developing. In other words, although the physiological conditions would seem to admit of the maximum rate of growth over the entire cut-edge, this only takes place in those parts that give the new tail its characteristic form. The growth in other regions is held in check. The same explanation applies to the more rapid growth at that part of an oblique cut that is nearest the base of the tail, for by this means the tail more nearly assumes its typical form.
These results demonstrate some sort of a formative influence in the new part. We can refer this factor at present only to some structural feature that regulates the rate of growth. We find here one of the fundamental phenomena behind which we cannot hope to go at present, although it may not be beyond our reach to determine in what way this influence is carried out in the different parts. This topic will be more fully considered in a later chapter.
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.
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.
Another experiment on the same animal gives also a somewhat similar result. If a worm is cut in two obliquely ([Fig. 21], B) and the regeneration of the posterior piece is followed, it is found that the new material appears at first evenly along the entire cut-surface. It then begins to grow faster on one side ([Fig. 21], b), and a head appears in this region with its axis at right angles to the cut-edge. As the head grows larger the growth is more rapid on one side, and as a result the head is slowly turned forward ([Fig. 21], b). This more rapid growth on one side brings the new head finally into its typical position with respect to the rest of the piece. The end result of these changes is to produce a new worm having a typical form. If the oblique cut is made behind the old pharynx, as in [Fig. 22], A, the new pharynx that appears in the new material along the cut-edge lies obliquely at first, indicating that the new median line is very early laid down in the new part, and connects the middle line of the old part with the middle of the new head. As the region behind the new head grows larger and broader the pharynx comes to lie more and more in an antero-posterior direction, and finally, when the new part is as broad as the old,[62] the pharynx lies in the middle line of a symmetrical worm.
These results show that the new growth may even take place more rapidly on one side of the structural median line than on the other, and on that side that must become longer in order to produce the symmetrical form of the worm. Here also we find that a formative influence of some sort is at work that regulates the different regions of growth in such a way that a typical structure is produced. The more rapid growth on one side is, however, in this case clearly connected with the relatively smaller development of the organs on that side, and perhaps this same principle may explain all other cases. If so the phenomenon appears much less mysterious than it does when the growth is referred to an unknown regulative factor.
DOUBLE STRUCTURES
A structure that is single in the normal animal may become double after regeneration, and in some cases the special conditions that lead to the doubling have been determined. Trembley showed that if the head of hydra is split lengthwise into two parts, each part may complete itself and a two-headed form is produced. If the posterior end of a hydra is split, an animal with two feet is made. It is true that the two-headed forms may subsequently separate after several weeks into two individuals, and even the form with two feet may lose one of them by constriction, as Marshall and King have shown. Driesch has produced a tubularian hydroid with two heads by splitting the stem partially into two pieces. Each head is perfect in all respects, and although each has fewer tentacles than