1. If a young gastrula of Discoglossus pictus is cut in half along the frontal plane the dorsal half develops into the anterior part of an embryo of normal size, there being no regulation; this is probably dependent on the failure of the wound to heal. The ventral half develops as a "Bauchstuck" of the usual kind, containing endoderm, mesenchyme, with occasionally some somitic mesoderm, enclosed in a bag of ectoderm.

2. Gastrulae were cut in half along the frontal plane, and the greater part of a second gastrula, from which the dorsal and ventral regions had been removed, was inserted in the plane of the cut (Fig. 1). Complete regulation occurred in many cases, leading to the production of abnormally large embryos. In most cases, some regulation occurred, but some ectoderm was left in excess and formed an excrescence near the anus.

3. The extent of the invagination of mesoderm is determined by an interaction between the presumptive mesoderm and presumptive ectoderm.

Part of the expenses of this investigation were defrayed by a grant from the Rockefeller Foundation, for which I should like to express my gratitude.