Regeneration in Animals

intimate, special, gradients, structure and tail

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Physiological Gradients.

Regions of active growth may be recognized by high rates of metabolism and physiological gradients made apparent by placing organisms into solutions which change colour on being acted on by the products of metabolic processes. (See AXIAL GRADIENTS.) In the lower animals, with high re generative capacities, these gradients are seen to be directed from the head backwards, so that some authors have ventured to attribute the reversion of polarity in cases of heteromorphosis to a high gradient arising through isolation of the head-piece at its new free surface in connection with the higher metabolic rate induced by the external stimuli. But it has not been possible to identify the head-formation with a special magnitude of metab olism, and as a second gradient is normally present passing from the tip of the tail forward it does not seem as if the special forms of head or tail could be explained by one sort of gradient.

Intimate Structure.

It seems necessary to postulate dif ferences of a qualitative sort in the intimate structure of the body regions to explain the different result, when an anterior or when a posterior piece has been isolated and has in the first case pro duced a reversed head, in the second a reversed tail, although in both cases the exposure to external stimuli was the same. It may well be that the physiological gradients are set up by differences in this intimate structure. It is also clear that fields of force must arise from a heterogeneous source if they are to be able to produce such complicated forms as we see developing in the long axis and in the members of organisms. What remains to be discovered is

how such an intimate structure is able to evolve from protoplas matic cells which must at the beginning (for instance in the egg) contain all potentialities, as experiments with division of eggs producing several whole embryos show. (See EXPERIMENTAL EMBRYOLOGY.) Thus there is a gradual decrease in regenerative potencies with increasing development, whether this be taken as the succession of different stages during the ontogeny of an animal or as the differentiation of regions from the centre of the body towards its periphery or even the attainment of a higher level during evolution of the species. The underlying principle may then be stated as a differentiation of special potencies from an originally unspecialized mass. If we consider different formative processes going on at the same time with different velocities in the developing organism, we have still to account for the orienta tion the structures show towards each other, when they arise by the various morphogenetic phenomena proceeding with unequal velocity in different directions of the body. This problem, how ever, does not need a special solution in the realm of regeneration, as here we are always dealing with masses showing in part the definite orientation of the axes in the animal. There can be no doubt that these already definitely oriented systems induce the blastemas of regeneration to conform to their scheme. (See also

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