Problems of Genetics

by William Bateson

In another region of observation phenomena occur which as it seems to me put it beyond question that the meristic forces are essentially independent of the materials upon which they act, save, in the remoter sense, in so far as these materials are the sources of energy. The physiology of those regenerations and repetitions which follow upon mutilation supplies a group of facts which both stimulate and limit speculation. No satisfactory interpretations of these extraordinary occurrences has ever been found, but we already know enough to feel sure that in them we are witnessing indications which should lead to the discovery of the true mechanics of repetition and pattern. The consequences of mutilation in causing new growth or perhaps more strictly in enabling new growth to take place, are such that they cannot be interpreted as responses to chemical stimuli in any sense which the word chemical at present connotes. Powers are released by mutilation of which in the normal conditions of life no sign can be detected. All who have tried to analyse the phenomena of regeneration are compelled to have recourse to the metaphor of equilibrium, speaking of the normal body as in a state of strain or tension (Morgan) which when disturbed by mutilation results in new division and growth. The forces of division are inacessible to ordinary means of stimulation. Applications, for example, of heat or of electricity excite no responses of a positive kind unless the stimuli are so violent as to bring about actual destruction.[6] These agents do not, to use a loose expression, come into touch with the meristic forces. Changes in the chemical environment of cells may, as in the experiments of Loeb and of Stockard produce definite effects, but the facts suggest that these effects are due rather to alterations in the living material than to influence exerted directly on the forces of division themselves.

By destruction of tissue however the forces both of growth and of division also may often be called into action with a resulting regeneration. Interruption of the solid connexion between the parts may produce the same effects, as for example when the new heads or tails grow on the divided edges of Planarians (Morgan), or when from each half embryo partially separated from its normally corresponding half, a new half is formed with a twin monster as the result.

Often classed with regenerations but in reality quite distinct from them are those special and most interesting examples where the growth of a paired structure is excited by a simple wound. Some of the best known of these instances are presented by the paired extra appendages of Insects and Crustacea. Some years ago I made an examination of all the examples of such monstrosities to which access was to be obtained, and it was with no ordinary feeling of excitement that I found that these supernumerary structures were commonly disposed on a recognizable geometrical plan, having definite spatial relations both to each other and to the normal limb from which they grew. The more recent researches of Tornier[7] and especially his experiments on the Frog have shown that a cut into the posterior limb-bud induces the outgrowth of such a pair of limbs at the wounded place. Few observations can compare with this in novelty or significance; and though we cannot yet interpret these phenomena or place them in their proper relations with normal occurrences, we feel convinced that here is an observation which is no mere isolated curiosity but a discovery destined to throw a new light on biological mechanics. The supernumerary legs of the Frog are evidently grouped in a system of symmetry similar to that which those of the Arthropods exhibit, and though in Arthropods paired repetitions have not been actually produced by injury under experimental conditions we need now have no hesitation in referring them to these causes as Przibram has done.

At this point some of the special features of the supernumerary appendages become important. First they may arise at any point on the normal limb, being found in all situations from the base to the apex. Nor are they limited as to the surface from which they spring, arising sometimes from the dorsal, anterior, ventral, or posterior surfaces, or at points intermediate between these principal surfaces.

With rare and dubious exceptions, the parts which are contained in these extra appendages are only those which lie peripheral to their point of origin. Thus when the point of origin is in the apical joint of the tarsus, the extra growth if completely developed consists of a double tarsal apex bearing two pairs of claws. If they arise from the tibia, two complete tarsi are added. If they spring from the actual base of the appendage then two complete appendages may be developed in addition to the normal one. We must therefore conclude that in any point on a normal appendage the power exists which, if released, may produce a bud containing in it a paired set of the parts peripheral to this point.

Fig. 11. Diagrams of the geometrical relations which are generally exhibited by extra pairs of appendages in Arthropoda. The sections are supposed to be those of the apex of a tibia in a beetle. A, anterior, P, posterior, D, dorsal, V, ventral. M¹, M² are the imaginary planes of reflexion. The shaded figure is in each case a limb formed like that of the other side of the body, and the outer unshaded figures are shaped like the normal for the side on which the appendages are. On the several radii are shown the extra pairs in their several possible relations to the normal from which they arise. The normal is drawn in thick lines in the center.

Next the geometrical relations of the halves of the supernumerary pair are determined by the position in which they stand in regard to the original appendage. These relations are best explained by the diagram (Fig. 11), from which it will be seen that the two supernumerary appendages stand as images of each other; and, of them, that which is adjacent to the normal appendage forms an image of it. Thus if the supernumerary pair arise from a point on the dorsal surface of the normal appendage, the two ventral surfaces of the extra pair will face each other. If they arise on the anterior surface of the normal appendage, their morphologically posterior surfaces will be adjacent, and so on.

These facts give us a view of the relations of the two halves of a dividing bud very different from that which is to be derived from the exclusive study of normal structures. Ordinary morphological conceptions no longer apply. The distribution of the parts shows that the bud or rudiment which becomes the supernumerary pair may break or open out in various ways according to its relations to the normal limb. Its planes of division are decided by its geometrical relations to the normal body.

Especially curious are some of the cases in which the extra pair are imperfectly formed. The appearance produced is then that of two limbs in various stages of coalescence, though in reality of course they are stages of imperfect separation. The plane of "coalescence" may fall anywhere, and the two appendages may thus be compounded with each other much as an object partially immersed in mercury "compounds" with its optical image reflected from the surface.