Orientation.—Applying the technique of grafting to animals (see GRAFTING IN ANIMALS) it has been possible to test experi mentally whether a structure already determined to form a special organ can be influenced by the place to which it has been trans planted, in regard to its orientation. If regenerating limbs are simply left growing, nobody would expect them to be oriented differently from the original limb they are replacing, therefore no special interest attaches to the fact that the regenerates always are in accord with the whole animal in respect to their orientation. But when young regenerating buds were first grafted without their fate being known, the question arose if a regulation would set in to give the bud the proper orientation towards the host independently of the way in which it had been artificially oriented to the host's stump. The answer seemed to be in the affirmative, when it was found that not only the orientation but also the differentiation itself conformed to the place where the graft had been attached. Only when older buds yielded the result already mentioned, was the observation made that with the determination to form a spe cial organ the orientation had also been stamped on them. When, for instance, an old regenerating bud is taken from a newt's limb it will develop a dorsal surface looking ventrally if it has been grafted upsidedown ; such a limb will, of course, be of no use to its bearer. Any sort of oblique and twisted limb can thus be pro duced by transplanting at different angles of rotation. The direc tion of growth always remains restricted to the long axis which runs proximodistally along the member. It remains restricted, but it need not proceed in the original direction. If a bud, already determined as to the sort of differentiation it is to undergo, be taken from a regenerating limb, its tip be cut off and this new cut surface adapted to the stump of the host's limb, it is of course prevented from growing in the right direction by the contact of the stump. Its free surface now grows in an opposite direction and something occurs that could be taken for a regulation : the differentiation proceeds as if the body were dictating the wanted orientation. Yet this is only apparent. One need only to change the dorso-ventral orientation, together with the reversion of the bud, to find the lack of influence on the part of the body as a whole, for no regulation towards the normal position will assert itself regarding the limb's back and palm. Further insight is given by experiments in which regenerating buds are not severed at their base but amputated together with a portion of old tissue. In grafting, this latter tissue is fixed to the host-stump and regen eration always proceeds along the lines originally laid down by the orientation of the old portion of tissue. Not only does this apply to cases in which the regenerating bud was old at the time of grafting, but also to young buds. These must then be determined by the orientation inherent in the old tissue immediately adjoining the regenerating cells, no influence from the body of the host com ing into play. Moreover, it is not necessary that any regenerating bud be present at the time of transplantation for the same result to appear. When a portion of a newt's limb is grafted in a reversed position, so that the originally distal cut surface becomes prox imal, outgrowth of all distal joints of the member will occur at the free surface, which before the amputation had been in con tact with the proximal part of the limb. Here again, since the superfluous leg is only orientated by the direction of its original outgrowth and free end, we see that there is no regulation exerted by the body itself. If we want then to arrive at an explanation of the phenomena in reversed grafts, it is necessary to take into account the faculties residing in the parts and set free, when opportunity arises, of growing in the opposite direction by the ex posure of cut surfaces. Not all parts are able to keep living when they have been totally separated from the animal, so that in many cases it is necessary to resort to grafting for keeping the parts alive long enough to watch their behaviour in regeneration. In these cases it is often difficult to decide if a given result has been influenced by the host or if the graft has exclusively used its inherent faculties.
Heteromorphosis.—On the other hand, animals which may be cut into several parts transversely to the length of the body with out dying, afford the opportunity of observing structures growing from cut surfaces in a reversed direction. Let a flatworm be cut into three pieces, the head-end with the eyes, the body to where it narrows in to the tail, and the tail. Then the middle piece will regenerate a head with eyes at its anterior, and a tail at its posterior cut surface. The head-piece will not regenerate a tail posteriorly but a second head will appear directed backwards. Similarly, the tail will grow no head anteriorly but develop a second tail. This example of the flatworm shows conclusively that the faculty of producing a structure opposite in direction to that of normal growth is inherent in the head-piece and in the tail-piece independently of the organism as a whole, for the latter is not present in the experiment. Similar results have been obtained with hydroids and also with annelid worms.
The regrowth of tissue, which does not exhibit the form of the missing part but substitutes another familiar form of the same species instead, is termed heteromorphic regeneration (see HETEROMORPHOgIS) ; reproduction of the opposite body-end is called axial heteromorphosis. When a transverse cut is made near the eyes of a flatworm without totally severing the head from the body, the latter may remain attached to the trunk by a small connection, the borders of the wound gaping. From each of its
free surfaces a bud of regeneration will arise. The one from the anterior margin of the trunk will of course develop a head looking in the same direction as the old one. But the bud from the posterior margin of the head behaves as if it were arising from a piece totally severed from the body, developing into a head point ing in the opposite direction to the other two. By taking a tri angular piece out of the body-side near the eye, the tip of the triangle reaching in to the middle line, the two cuts will again try to produce heads. These will grow at right angles to the edges of the wound, but not having room enough they will impede each other in their development and more or less coalesce.