GENERATION ; GRAFTING IN ANI MALS.) It will be seen that several di verse processes are at present lumped together under the head of dedifferentiation. Not only is the dedifferentiation correlated with increased multiplicative ac tivity to be sharply distinguished from that correlated with de pression of activity, but among depressant agencies starvation, at least in moderate degree, proba bly has a different, less pathological effect than exposure to chemically unfavourable conditions. Distinction should also be made between reversible dedifferentiation and that which is irre versible and therefore leads to degeneration (though reversible dedifferentiation, if long continued, often passes over into irrevers ible). In reversible cases, investigation is needed as to whether the dedifferentiated cells themselves redifferentiate (as in Proto zoa, and undoubtedly in some Metazoan cases, e.g., in early stages of Clavellina's dedifferentiation), or whether they degenerate, and redifferentiation occurs from undifferentiated "reserve" cells. In vestigation is also needed with reference to "metaplasia"—the capacity for tissues to transform from one differentiated type into another. While this undoubtedly occurs, it is probably confined to the power of a tissue to pass from lower to higher grade of differentiation, as when non-cornified epidermis becomes converted into cornified under abnormal stimuli ; and to redifferentiation in a new direction after passing through a dedifferentiated phase in which cell-multiplication has taken place, as in the above-cited example from regeneration. The study of tissue-culture may solve several of these problems.
See also REGENERATION, GRAFTING IN ANIMALS, CANCER, TIS SUE-CULTURE.
Schultz, "tYber umkehrbare EntwicklungsproBibliography.-E. Schultz, "tYber umkehrbare Entwicklungspro- zesse," Roux' Vortriige and Aufsiitze fiber Entwicklungsmechanik (1908) ; C. M. Child, Senescence and Rejuvenescence (1915) ; B. Darken, Einfiihrung in die Experimentalzoologie (1919) ; T. S. Huxley, Quart. J. Micr. Sci. (1921) ; Morley Roberts, Malignancy and Evolution (1926). (J. S. H.) DE DONIS CONDITIONALIBUS: see ENTAIL. DEDUCTION, a term used in common parlance for the process of taking away from, or subtracting (as in mathematics), and specially for the argumentative process of arriving at a con clusion from evidence, i.e., for any kind of inference (from Lat. deducere, to take or lead from or out of, derive) . Two forms of the verb are used, "deduce" and "deduct" ; originally synonymous, they are now distinguished, "deduce" being confined to argu ments, "deduct" to quantities. In this sense it includes both ar guments from particular facts and those from general laws to particular cases. In logic it is generally used in contradiction to "induction" for a kind of mediate inference in which a conclusion (often itself called the deduction) is regarded as following neces sarily under certain fixed laws from premises. This, the most common form of deduction, is the syllogism (q.v.; see also LOGIC), which consists in taking a general principle and deriving from it facts which are necessarily involved in it. This use of deduction is of comparatively modern origin ; it was originally used as the equivalent of Aristotle's aoraywyri (see Prior Ana lytics, B xxv.). The modern use of deduction is practically iden tical with the Aristotelian ovXXoycaµos. Logical usage is some what inconsistent. On the one hand, Deduction is said to be from a universal premise ; on the other hand, even syllogisms consisting of singular propositions only are described as deductive. To se cure consistent usage it is best to apply the term deduction to all inferences from a universal proposition (even to immediate in ferences of a singular or particular proposition from a universal) and to no other inference (such as singular syllogisms). Another source of confusion lies in the fact that in Mathematics the term deduction is sometimes used as synonymous with Analysis. Des cartes' "deductive method" is often misunderstood for this reason as it covers both deduction proper and this analytic method.
