Summary1. Amphibian eggs are spherical, while the embryos are bilaterally symmetrical. The latter is manifested morphologically when gastrulation begins with the formation of the blastopore at a bilaterally symmetrical (vegetal‐dorsal) location on the surface of the embryo. To account for this change in symmetry two polarities (vectors or axes) are required. These need not go through the centre, but if they do, one will go through two poles, called ‘animal’ and ‘vegetal’ in the amphibian embryo, and the other will pass through two points on opposite sides of the egg, one at the ‘dorsal’ and one at the ‘ventral’ side. Together these two polarities define a plane of bilateral symmetry.2. It may be assumed that one polarity determines that gastrulation begins in the vegetal hemisphere, and the other that it begins at the dorsal side.3. Judging from the distribution of pigment in the cortex of the egg and that of the yolk‐hyaloplasm in the interior, an animal‐vegetal polarity is already present in the unfertilized egg. That cytoplasmic components are actually part of the material substrate of this polarity is evident from the fact that the pattern of gastrulation may be upset if the distribution of yolk‐hyaloplasm is deranged.4. At fertilization the pigment border is raised at the side opposite the fertilizing sperm, giving rise to the ‘grey crescent’. The latter confers the first visible bilateral symmetry on the egg, and in fact it determines the presumptive median plane, for blastopore formation begins in the midline of the grey crescent.The dorso‐ventral polarity imposed by the sperm is not irreversibly determined. By various experimental means, e.g. restriction of the oxygen supply, it may be inverted.5. In order to understand the mechanism of the polarities it is necessary to study the processes on which the effects of the polarities are exerted, viz. the process of invagination associated with the formation of the blastopore. It has been known for a long time that at the bottom of the blastoporal groove are located some large flask‐shaped cells, called ‘Ruffini's cells’. Various arguments can be mobilized to support the notion that these cells actually are engaged in pulling in the embryonic surface.6. These cells are the first representatives of a cell type different from the spherical cells which are typical of the early embryo. It may therefore be presumed that Ruffini's cells are the products of the first cell differentiation occurring during amphibian embryogenesis. And it may further be assumed that the polarities somehow control this process.7. A number of observations suggest that the animal‐vegetal polarity is in direct control of the differentiation, ensuring that Ruffini's cells are formed only in the vegetal hemisphere. This point has been corroborated by isolating in cultures small aggregates from various regions of the blastula. When this is done it is found that the only path of differentiation available to animal cells is the formation of small spherical aggregates composed of a mixture of ciliated and non‐ciliated cells. In contrast, in cultures of vegetal cells an outgrowth of cells occurs, and these cells share a number of properties with Ruffini's cells, and it is suggested that they are representatives of this cell type.8. The formation of these cells is suppressed by inhibitors of RNA synthesis and by anaerobiosis induced by KCN. Since oxidative metabolism is apparently required for the differentiation of Ruffini's cells ‐ gastrulation in the intact embryo is suppressed by anaerobiosis ‐ a number of carbohydrate metabolites were scrutinized for their effect on the formation on Ruffini's cells. It was found that at 10 mm lactate completely suppresses their appearance, and indeed all the other cell differentiations that can otherwise be observed in our cell cultures. Since there is a very steep animal‐vegetal cytoplasmic gradient in carbohydrate, the content being lowest at the vegetal pole, lactate might potentially be the agent of the animal‐vegetal polarity, but there are a number of facts which do not readily support this idea.9. If animal cells are explanted together with a few vegetal cells, some of the aggregates do not become ciliated, but rather exhibit an outgrowth similar to the one observed with vegetal cells. These animal cells have the same general shape as the vegetal Ruffini's cells, but they are smaller and more pigmented, typical ‘animal’ features. When the cultures are preserved, the cells undergo further differentiation, becoming either ‘mesenchyme’ cells, nerve cells, pigment cells and sometimes even muscle cells may be observed. In the normal embryo these differentiation patterns occur in that part of the animal hemisphere which becomes induced through contact with the vegetal material entering the blastocoel during gastrulation. Thus there is reason to assume that the induction occurring in our cultures is a miniature of the normal induction process.10. Just as in the sea‐urchin embryo, the animal cells in amphibia may become ‘vegetalized’ by addition of Li+to the culture medium.11. For various reasons it is likely that Ruffini's cells contain heparan sulphate, and in the belief that this substance might be the inductor proper, its effect was tested on animal cells. It turned out that in a concentration of 0·1 ppm it can alter the differentiation pattern of these cells, and we suggest that heparan sulphate, for the time being, is the most likely candidate for the role of primary inductor in the amphibian embryo.12. The edges of the blastoporal groove, and hence the formation of Ruffini's cells, proceeds gradually around the circumference of the embryo. The effect of the dorso‐ventral polarity therefore appears to be concerned with the time at which the cells undergo differentiation, imposing a spatial and a temporal gradient on this phenomenon. The second overt manifestation of the dorso‐ventral polarity, next to the formation of the grey crescent, concerns the size of the embryonic cells, the dorsal ones being always smaller than the ventral. This fact suggests the possibility that the polarity may exert its effect by interfering with the process of cell division.13. The cell divisions in the early embryo are distinguished by being synchronous; all cells are either undergoing mitosis or they are in interphase. The duration of the latter is typically very short. After a certain number of cell divisions, around 10, when the embryos are in the mid‐blastula stage, the synchrony is gradually lost, while the interphase becomes considerably prolonged. This peculiar behaviour suggests that the cytoplasm of the early embryonic cells contain some factor which ensures the synchrony. The well‐known presence in the early embryo of deoxyriboside‐containing material, in an amount corresponding roughly to the total amount of DNA residing in the cell nuclei after 10 cell divisions hinted that deoxyribosides might indeed be the ‘synchrony factor’.14. This idea was tested first on intact embryos. An excess of deoxyribonucleotides was injected into very early embryos. The result was developmental arrest at a pregastrula stage (no Ruffini's cells formed) in a large percentage of embryos. However, the number of cells was greater than in the controls, and the rate of cell division higher, indicating a delay in the transition to synchrony, thus supporting the proposed mechanism.Furthermore, the deoxynucleotides inhibited cell differentiation and an explanation of this was found in the fact that they also strongly inhibited RNA synthesis.15. The studies were extended to cell cultures. It was found that deoxyribosides inhibit the differentiation of animal as well as vegetal cells; instead, the cells go on dividing at least for another two rounds. The utilization of added deoxyribosides does not demonstrate that the endogenous substances are similarly utilized. That they are, was indicated by the following experiment: In the presence of cytosine arabinoside, an inhibitor of DNA synthesis de novo, the explanted cells go on dividing an unknownferentiation. But in either case these cells are larger (about four times) than the controls. This result suggests that in the experimental cultures the cells go on dividing as long as the cytoplasmic deoxyribosides last and then stop, while the controls synthesize their own DNA for two rounds of division before they undergo differentiation.16. It is now possible to suggest a mechanism for the dorso‐ventral polarity. First it affects the cell size such that the dorsal cells are the smallest. If the cytoplasmic deoxyribosides are evenly distributed at the outset, then small cells must be nearer exhaustion than large ones. A dorso‐ventral gradient in cell sue will therefore automatically imply a dorso‐ventral gradient in the time at which the cells reach the state in which they can undergo differenti