Loeb uses the word 'heteromorphosis' to denote the ability possessed by organisms, under the stimulus of external forces, to produce organs on parts of the organism where such do not occur normally, or the power to replace lost parts by parts unsimilar to them in form and function. Regeneration is the reproduction of parts like those lost; heteromorphosis is the reproduction of parts unlike those lost.
Heteromorphoses are well known in plant physiology. When one cuts a slip from a willow, one may make the cut at the bottom of the slip and the cut at the top in any part of the willow-twig, yet still the lower end of the slip always produces rootlets, which are organs not normal to that part of the twig, while shoots will rise from the upper end. Moreover, either end of the slip may be made the root portion, and it is clear, therefore, that in every small area there are cell-groups present able to bear roots or shoots according to the determining conditions; and therefore that, in addition to the characters active at any time, there are present the germinal rudiments for shoots and roots, and, indeed, for the whole organism, since the shoots ultimately may bear genital products.
When the prothallus of a fern has developed normally, it is a flattened leaf-like structure which bears rootlets and male and female genital organs on the lower surface, i.e., on that turned from the light. But the experimenter may reverse this order, by artificially shading the upper surface, and strongly illuminating the lower surface.
Among the most interesting heteromorphoses are the galls, produced upon young plants when certain insects lay eggs on them, or when plant-lice irritate their tissues. From these abnormal stimuli there result active masses of cells which grow into organs of definite form and of complex structure. The galls, moreover, differ widely, in correspondence with the specific stimulus which was their initial cause, and with the specific substance, the stimulation of which resulted in the formation of a gall. By the action of different insects upon the same plant different galls are produced, and the galls of different plants may be distinguished systematically.
Blumenbach has already brought forward the existence of galls as an argument against preformation, holding them to be structures produced epigenetically, and, therefore, unrepresented by rudiments in the germ. I, also, consider them witnesses against Weismann's germplasm. They teach us that the cells of the plant-body may serve purposes quite different from those arranged for in the course of development; that cells modify their form in correspondence with novel conditions, and that they are forced into forming special structures, not by special determinants in the germ, but by external stimulants.
Galls exhibit yet another instructive kind of heteromorphosis.
Even the tissue of a leaf, turned into a gall by pathological conditions, retains the power of producing roots. Beyerinck has shown that galls of Salix purpurea, planted in moist earth, bear rootlets identical with those of the normal plant. As the roots of all woody plants are able to bear adventitious buds, De Vries thinks it probable that one could rear a whole willow-tree from a gall. That would imply that all the inheritable characters of the willow were contained even in the gall.
Loeb has produced heteromorphoses experimentally upon many lower animals, among which were Tubularia, Cerianthus, and Cione intestinalis.
In Tubularia mesembryanthemum, a hydroid polyp, there are stalk, root, and polyp-head. If one cut off the head, a new head will be formed in a few days, this being a case of regeneration. On the other hand, a heteromorphosis may be produced by modifying the experiment as follows: Both root and head must be cut from the stem; if the lopped piece of the stem be stuck in the sand of the aquarium by the end that bore the head, then the original aboral pole in a few days produces a head; if the lopped piece of stem be supported horizontally in the water, then each end of it produces a head.
In a Cerianthus membranaceus (Fig. 1), the body was opened by a cut some distance below the mouth, whereupon buds appeared on the lower edge of the slit, where the experimenter had prevented coalescent growth. These buds gave rise to inner and outer tentacles, and an oral disc was produced. Thus, artificially, an animal with two mouth-openings or two heads was produced; and, similarly, animals with a row of three or more heads may be produced.