The unilateral or bilateral extirpation of the ectoderm of the P.H.M. region in stages 5 to 8 in the chicken embryo exerts no influence in heart determination and differentiation, since in all the cases a morphological and structurally typical heart is formed. 2. Neither does the extirpation of the ectoderm interfere in the migratory process of the cells of the P.H.M., since, with the exception of one case of cardia bifida, a unique heart is formed in a unique pericardial cavity. 3. The extirpation of the ectoderm exerts a clear influence over the situation of the heart and over its curvatures since when the ablation of the ectoderm is unilateral, the heart is displaced toward the side opposite that of the extirpation. It is, therefore, normal in the extirpations of the left ectoderm and inverted in those of the right. 4. The primary fore-gut pocket does not form on the side where the ectoderm has been extirpated. This, therefore, exerts a clear influence on the formation of the fore-gut pockets and in the form the fore-gut takes. 5. The extirpation of the ectoderm in stages 6 and 7 constantly inhibits the process of the closing of the neural canal in the encephalic region (mesencephalon and anterior part of the rhombencephalon), the neural differentiation normally continuing its process. This suggests that the force of the neural folds during the formation of the neural tube is transmitted to them through the ectoderm.
