The clue was afforded by some crayfish, which normally possess claws of different size and form on the right and left side of the body. Instances of this heterochely occur in the pistol-crab Alpheus, the common lobster Homarus, the hermit-crab Pagurus, the shore-crab Carcinus, the swimming crab Portunus, the fiddler-crab Gelasimus and others. When the large crushing shaped claw is removed from a pistol-crab the small cutting claw of the opposite side will, after one or two moults, assume the size and form of its former opponent, whilst a small cutting claw regenerates in place of the latter. When the same loss has befallen a lobster this reversal in the type of claws, as we have observed, will not occur, a crushing claw arising directly by regeneration, the cutting claw remaining as it was. As the type of a given body-side is thus not changed in the claw, we are able to decide in triplicities of a lobster's claw, if one of the components belongs to the other body-side by its type. For tunately, this sort of monstrosity is not rarely found in lobsters and related species, so that comparisons can be made between supernumeraries on the crushing and cutting claws. On the latter they are always cutters, on the former typical crushers or transi tional forms. These latter are easily explained as an earlier stage in the development of every crushing claw. So we can conclude that not only the components with the asymmetry proper to the body-side on which the triplication has appeared, but also the one between these, notwithstanding its mirror-image orientation, is not to be regarded as a limb belonging to the other side, but simply as a proximal outgrowth from a wound of the claw on the same side. The mirroring is accounted for by the anterior margin of the original claw always giving rise to an anterior margin again, the posterior to a posterior one, the dorsal to a dorsal, the ventral to a ventral. Although this example and most regenerations after amputation of parts or whole members by transverse cuts seem to indicate that not only will every tissue produce its own kind, but also the special differentiation it has undergone itself, such a sweeping statement would not be in accord with other facts.
We have already treated of cases in which a dual faculty is revealed by heteromorphic regeneration. Another classical object is the lens of the newt's eye, which during em bryological development is formed by the infolding of external skin, but if extracted later is replaced by regeneration from the iris. When bones are totally removed from limbs of newts or salamanders, leaving all the other parts in place, no regeneration of the excised bone is observed. But if now the limb is severed, the plane of amputation passing where the distal end of the re moved bone should have been a blastema of regenerating cells will make its appearance. Although no bone is present at its base, this blastema will not only regenerate the general form of the limb, but also all the bones which normally lie distal to the plane of amputation. It is then clear that we have long been cheated by the coincidence of all tissues being present at a cut surface and the orientation of the margins remaining constant to believe in a necessity of the presence of tissues for regeneration which in reality may arise anew in the undifferentiated cells of the re generation-bud. Remembering the use lower animals make of
interstitial cells for restoring their form, we are again led to the conclusion that indifferent material is moulded by a formative agency. It is at present not possible to decide what category of force may be involved. Therefore, in order not to bias further investigation, the term field of force, as used by physicists and chemists, has been applied to regions where formative influences are at play in the organism. Many views have been taken as to the nature of these influences, starting from a crude predetermina tism and merging into conceptions of processes not acting point by point, but more after the manner of electric or magnetic in duction, every point of the system being connected with the other by quantitative relations not altered when the size is changed. It has been pointed out how this view harmonizes with the re generation from a cut surface after extraction of the bone or when a complete structure appears at the end of both pacts of a split wound.
Other facts apparently supporting this conclusion have been found in defective or incomplete re generation. We have till now only been describing new formations in which either the loss has been totally repaired or another struc ture taken its place or, lastly, supernumerary parts have arisen. Yet there are typical regenerative phenomena in which none of these possibilities are realized, the restorative process stopping before completion. The earthworm, when its head-region is am putated, does not regenerate as many segments as have been cut off. Stick-insects and other orthoptera with five-jointed tarsi in their limbs replace as a rule only four joints of ter autotomy. In water-fleas (Daphnia), joints in the second antenna are omitted during regeneration. Now in all these instances the terminal endings are always present, the defect resulting from the omission of basal segments. This also occurs in the newt's leg when in sufficient material is available for regeneration. This has been especially observed when small regeneration buds are transplanted to another field of regeneration. If the regenerate were growing step by step we would rather expect the basal structures to be present, the distal to be inhibited, when there is a shortage of material. A field of force, on the contrary, could be imagined first laying out the margins and filling up the gaps as best possible afterwards. Captivating as this appears, yet there are other modes of explanation. If we take the direction of growth into account, we find that in the cases cited normal growth does not proceed all the time from the base towards the apex but reverses its direction after the most basal segments have been formed, so that the apex has grown before the segments just in front of it have developed. Therefore the segments omitted in regeneration are those which ought to be last constricted off and would be stopped first in case of deficiency. It is then not necessary to invoke a special action of the field as a whole.