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synopsis. The theory of differentiation during regeneration by polarized specific repression in the bioelectric field as worked out in Tubularia is reviewed and brought up to date. Similarities between the studies with Tubularia and those on a variety of other organisms are discussed. The major question being asked is, "How does a cell learn from contacts with its neighbors which of its special genes it should use?" It is generally agreed that all of the cells of an organism possess all of the genes. Differentiation occurs as each cell comes to use just a few of its special genes. The major part of the work being re? ported was done on the hydroid, Tubu? laria, at the Marine Biological Laboratory, Woods Hole, Mass. If a piece of stem of Tubularia 5 mm or longer from any level of the stem is iso? lated, the anterior part of it will transform to a hydranth in 1 to 2 days. When shorter pieces, too short to produce a whole hy? dranth, are isolated, they may transform to part of a hydranth. When only a part of a hydranth regenerates, it is always the an? terior part. In thousands of cases never have pieces too short to produce whole hy? dranths produced anything other than anterior portions of hydranths. These ob? servations tell us that the genetic informa? tion and the means to transcribe and

After hydranth removal of Tubularia the level of DNA synthesis, measured by nuclear incorporation of thymidine-H3. is uniform throughout the caulus from the time of hydranth amputation through wound healing. Synthesis is then suspended during massive cell movements until the primordium begins to form; activity resumes in both the ectodermis and endodermis in all subsequent regenerative stages with qualitative and quantitative differences along the caulus. Incorporation is highest in the proximal ectodermis and grades off distally; in the endodermis uptake increases in a distal direction and becomes highest in the primordium. As differentiation continues, synthesis appears in the future gonophore region, the inter-tentacular zone, and the developing tentacles. When differentiation is completed, further uptake occurs in the peduncle, basal hypostome, and both cell layers of the tentacles. Monitoring the movement of hydrocaular cells, labeled at amputation, confirms that there is a general shift of the coenosarc along the entire length of the caulus toward the distal end. Cells continue to migrate from both cell layers of the caulus into the hydranth primordium even after it is formed. Labeled proximal cauli grafted to unlabeled distal halves indicate that cells from the ectodermis and endodermis still contribute to the hydranth regenerate. There is no evidence for reverse cell movement. Incorporation of thymidine-H3 (suspended during massive cell movements) does accompany individual cell migration.

It is a rather strange coincidence that so many different factors have similar effects in either inhibiting or accelerating the onset of metamorphosis in widely divergent groups. Of these factors the heavy metals seem to play a distinctive role. Copper, for instance, hastens the beginning of metamorphosis in several ascidians (Grave and Nicoll, 1939; Glaser and Anslow, 1949), Bugula larvae (Miller, 1946; Lynch, 1949), the coelenterate, Tubularia, (Pyefinch and Downing, 1949) and oysters (Prytherch, 1934; Armstrong and Miall, 1945) ; but the effect of copper on oyster larvae has not been confirmed by others, and Armstrong and Miall (1945) do not give experimental data on the copper ion. Clark (1947) found that sublethal exposures to cupric citrate accelerated the growth of newly metamorphosed barnacles, but he did not state that the metal hastens attachment. Glaser and Anslow

Sectioning experiments and electrical recording indicate that there are many potential pacemakers in polyps of the hydroid Tubularia . Functionally the pacemakers are organized primarily into pacemaker systems, groups within which there is tight coupling. The different pacemaker systems of a polyp are loosely coupled to one another. There are two principal systems in Tubularia , one in the polyp neck (the NP system) and one in the hydranth (the HP system). In addition, there are pacemakers controlling activities of individual tentacles. Activity in the NP system is usually not associated with observable polyp behavior. HP system activity is correlated with behavioral responses termed concerts. Concerts are probably digestive activities; they result in the mixing of food being digested and the distribution of the products of this digestion. The NP systems of polyps on a colony are loosely coupled to one another through one of the three conducting systems found in Tubularia stalks. The loose coupling between NP systems of polyps on a colony and the loose coupling between NP and HP systems within single polyps results in there being some coordination of concert activity throughout a colony.

The specific architecture of a Hydroid polyp or a colony is, morphologically speaking, not a stable entity, because it is subjected to an incessant turnover and renewal at both cellular and higher levels. A continuous morphogenetic activity is in many cases (e.g., Hydra ) necessary for the maintenance of the shape and size of the individual polyp, because it has to compensate an equally continuous loss of cells at the level of the tentacles and at the base of the polyp. Colonial species, such as Tubularia, Campanularia, and others, show regression-replacement cycles, some of which are conditioned by endogenous factors, while others are released by ecological factors, such as environmental stresses. In all these morphodynamic events, regeneration plays an important part. It is an expression of the morphogenetic plasticity of the hydroid polyp, just as is asexual reproduction, another category of morphodynamic elforts these organisms are subjected to.