ABSTRACT
THE WORD 'regeneration' is used to refer to the processes by which an animal restores, or tends to restore, any regions which may be removed. It covers a wide range of phenomena (General Review: A. E. Needham 1952). At one extreme an adult mammal which has suffered the loss of a small part, such as a finger, or a larger part, such as a limb, can do no more in the way of regeneration than merely repair the wound and close the cut surface. At the other extreme a very small part of the normal body of a coelenterate, a flat worm, or a starfish, can restore the whole large region which is missing and become a complete individual. There are all grades in between these two extremes. The power of regeneration is in general greater in lower forms and less in more highly evolved ones, but this rule is only very rough, and when one looks at the matter in more detail it becomes apparent that the capacity for regeneration is distributed in a rather arbitrary manner throughout the animal kingdom. Often even closely related forms differ considerably in their regenerative powers. The mass of information on the subject is very large and there would not be space here to review it completely. It is, however, necessary to take a glance at certain aspects of the subject, particularly because of the light it throws on two general points of embryological theory. These are, firstly, the reversibility of determination, and secondly, the field theory.
