Heart determination is a gradual, cumulative process involving inductive and suppressive interactions between the heart mesoderm and nearby embryonic tissues. Our analysis of heart determination in the California newt, Taricha torosa, includes defect and other i n vivo experiments, and explants in epidermal vesicles and into hanging drops.Explants of presumptive heart mesoderm from neurulae into hanging drops of a completely defined salt solution (Niu-Twitty solution) produce beating hearts only infrequently. The addition of various other tissues and fractions of tissue homogenates changes the frequency and the rate of differentiation. These two parameters were used to assess the stimulatory and suppressive effects of tissues and tissue fractions on the differentiation of the presumptive heart mesoderm.At least three different factors are active in eliciting and regulating heart differentiation. A specific heart inductor in anterior endoderm increases the rate and the frequency of heart differentiation. A general stimulatory factor in epidermis and other embryonic tissues increases the frequency, but not the rate, of heart differentiation. A n inhibitory agent in cranial fold and neural plate tissues delays or prevents heart differentiation. These three factors operate from intact tissues in the embryo or when explanted in vesicles or hanging drops, and are effectively present in a fraction (from sephadex column chromatography) of homogenates of the appropriate tissues.Heart formation in vertebrates is clearly dependent upon a variety of developmental conditions and processes. We shall describe some of these and attempt to integrate them into a unified concept of heart differentiation.By the time an embryo has formed the three germ layers, heart potentialities have become limited to, or have developed only in, specific regions of the mesoderm. Portions of the endoderm and the ectoderm have significant interactions with the mesoderm, and these interactions control heart formation. As with so many organs, the heart is induced.Prospective heart mesoderm has its most important inductive interactions with the anterior dorso-lateral endoderm of the early embryo. This has been experimentally demonstrated in amphibians (Mangold, '56; Chuang and Tseng, '57; Amano, '58; Jacobson, '60, '61), in chicks (Orts-Llorca, '63; Orts-Llorca and Gil, '65), and is implied for human embryos by observations of Hommes ('57).Partly as a result of embryonic induction, areas of mesoderm become determined to form a whole heart. For a time during heart determination, any part of such an area may form a whole heart if isolated from the remainder. This is readily seen if one prevents the fusion of the two groups that normally come together to form a single heart. Each half-rudiment forms a whole heart, one the mirror image of the other (Ekman, '25; Copenhaver, '26, '55; DeHaan, '65).The positions of the cells that are induced to form heart and the morphogenetic movements that bring these cells to the heart site need explanation. Atte...

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Trunk neural folds from both black and white axolotls, Ambystoma mexicanurn, were transplanted unilaterally in place of folds of Taricha torosa neurulae. Melanophores from either kind of axolotl completely suppress the differentiation of T. torosa melanophores for several stages and largely suppress it for much longer. During larval stages host pigmentation gradually develops, but axolotl pigmentation always dominates. After metamorphosis the typical Ambystoma spotted pattern appears. During all stages donor melanophores are almost equally distributed on both flanks, with only slightly more on the grafted side. Guanophores predominate on the grafted side, while xanthophores predominate on the opposite side. With the exception of the distribution of axolotl melanophores, these results confirm results of others with the black axolotl and offer additional evidence that the chromatophores of both black and white axolotls are equivalent. New suggestions are made to explain the unequal distribution of xanthophores and guanophores in view of the nearly equal distribution of axolotl melanophores.Trunk neural folds of T. torosa were transplanted bilaterally in place of white axolotl neural folds. The typical T. torosa larval pigment pattern is expressed in the grafted area, but host melanophores suppress donor melanophores beyond the confines of the graft. Eventually all axolotls immunologically rejected their neural crest grafts.Only 13% of the T. tmosa rejected their grafts. These results confirm that T. torosa has a much greater tolerance to grafted foreign tissue than do axolotls. The immunological behavior of these animals is discussed in the context of results of similar experiments in anurans.The phenomenon of the suppression of melanophore differentiation in one species of urodeles by the differentiating melanophores of another species has been known for a long time. Twitty and Bodenstein ('39) first observed this suppression in combinations of neural crest cells between the California newt, Taricha torosa, and two species of Ambystoma. Differentiating melanophores of both A. maculatum and A. tigrinum suppressed the differentiation of T. torosa melanophores when neural crest of the latter was grafted to either of the former during tail bud stages or when the reciprocal grafts were made. The more rapidly differentiating Ambystoma melanoblasts seemingly usurped all of the melanogenic elements of the tissue environment before the T . torosa melanoblasts were ready to differentiate. The grafting of older Ambystoma neural crest cells onto younger T. torosa increased the extent of the suppression, for under these circum-J. EXP. ZOOL., 163: 331-346. stances the Ambystoma melanophores were developing even earlier relative to the T . torosa neural crest cells (Twitty and Bodenstein, '44). Whenever Ambystoma neural crest was grafted to T . torosa, the Ambystoma melanophores suppressed those of T. tmosa not only in the region of the graft but also for some distance anterior and posterior to the graft.Using a new tech...