1. This article reviews the present state of knowledge on regeneration in the Hydrozoa and some of the problems related to it. The available data derived from descriptive and experimental studies are confined to only a few genera (Hydra, Tubularia, Cordylophora, Corymorpha, Campanularia, Pennaria). Without enumerating all possible types of regeneration, a number of the most significant cases are examined from various points of view.

2. Some of the anatomical and physiological particularities exhibited by the Hydrozoa have been emphasized as a possible basis for their remarkable morpho-genetic abilities. The diploblastic architecture of a hydroid polyp is of a relatively simple design, lacking–in contrast to the higher Metazoa–a number of structurally and functionally well-circumscribed organs. AH vital functions are distributed amongst a very small number of cell types, which have during their differentiation retained a relatively high standard of physiological autonomy and adaptability, being able to undergo, within certain limits, structural and functional transformation. Such differentiated somatic cells are subjected to normal senescence, which is compensated by a continuous (Hydra) or periodical (Tubularia) replacement activity. This rejuvenation process depends upon the presence of a permanent self-proliferating stock of indifferent replacement cells (interstitial cells).

3. In a single polyp or in a colony the quantitative pattern of the morphogenetic or regenerative potentials is inversely proportional not only to the degree of tissue differentiation (Needham, 1952), but also to the progress of physiological ageing. In this particular case the age of a tissue or body fraction is not determined so much by the age of the somatic elements as by the number of available undifferentiated replacement-cells. In the hydrocaulus of Tubularia the graded morphogenetic potentials are proportional to the degree of basipetal ageing of the coenosarc tissue.

4. The organogenetic pattern of a developing regenerate is constituted by differentiated somatic cells, which migrate to or are shifted towards the site of recon-stitution together with the undifferentiated interstitial cells. The function of organizer is delegated to the endoderm layer, while the interstitial cells provide the regenerate with additional young elements which differentiate according to their position in the pre-established organogenetic pattern. The quantitative behaviour of nucleic acids, free amino acids and proteolytic enzymes suggests that the structural differentiation of the regenerate is accompanied by a synthesis of new, probably organ-specific proteins.

5. The mechanism by which the regenerating system controls the morphogenetic activities includes an inhibitory principle. In fact the hydranths of various species were found to produce a factor which has the faculty of reversibly inhibiting the recon-stitution of an equivalent structure in the same morphogenetically active system. Within a developing regenerate the rate of production of this chemically unknown inhibitor and the degree of susceptibility are quantitatively equilibrated so as to exclude any inadequate self-inhibition. The low molecular substance seems to exert its inhibitory action by interfering with the oxidative systems of the regenerate. In connexion with the axial gradients this inhibitory principle establishes a hierarchy of morphogenetic dominance which avoids a chaotic outbreak of latent potentials and is responsible for the determination of axial polarity.

6. In the Hydrozoa regeneration is not only the appropriate response to accidental mutilation but it is an integrated element of the normal life cycle, participating in sexual and asexual reproduction as well as in normal growth. Many colonial hydrozoa periodically replace their short-lived hydranths by a process of regeneration which resembles strobilation (Tubularia). This regression-replacement cycle, besides serving other functions, constitutes a mechanism by which the colony periodically rejuvenates its terminal organs, as there is no regular exchange of cells between these and the colonial system.

7. Autotomy or resorption of hydranths, compensated later by regeneration, can also be provoced by ecological factors acting accidentally or with normal seasonal rhythms. Under unfavourable conditions the colonies–particularly those which are partly protected by an exoskeleton–avoid an intimate contact with the surrounding medium by liberating the exposed periferial parts (Tubularia) or by resorbing them and withdrawing the material into the more protected stolon system (Pennaria).