In stages +4, 5 and +5 in the chick embryo, the bilateral extirpation of the endoderm in the entire extension of the P.H.M. region detains heart differentiation, giving rise to acardiac embryos. 2. The unilateral extirpation of the endoderm in the entire extension of the P.H.M. region detains heart differentiation of the primordium of the side operated on, forming a typical beating heart from the heterolateral primordium. 3. If the extirpation of the endoderm is partial, a small beating vesicle is formed on the side operated on, which has the characters of the ventricle bulb or atrium, according to which region of the endoderm was extirpated. 4. It is not possible to demonstrate a difference between right side and left side extirpations of the endoderm. 5. The formation of the fore-gut is detained completely or unilateraly due to the absence of the morphogenetic movements of the endoderm in the extirpated region. Neither are the fore-gut fissures formed. 6. The embryos which form are abnormal in the encephalic extremity, observing platyneuria and vascular dilatations with great frequency. 7. In the stages in which these experiments have been carried out, the P.H.M. is not found to be determined as yet and still needs the influence or induction of the endoderm in order to initiate its differentiation.
