(1) Control series I. The ectoderm of the early gastrula of Triturus pyrrhogaster was isolated and exposed for a short time to a solution of ammonia and then cultured in Holtfreter solution. The isolate differentiated structures which can be characterized as cephalic ectodermal rudiments including brain, eye. nose and frontal glands. Otic vesicles, spinal cord and mesodermal structures were wholly absent. (2) Control series II. The ventral marginal zone of the early gastrula was isolated and cultured in Holtfreter solution. It differentiated blood island, blood vessels and mesothelium and nephric tubules. (3) Experimental series. The isolated ventral marginal zone was treated with ammonia solution and cultured in Holtfreter solution. It gave rise to notochord and muscle as well as nephric tubules. The development of the blood island, mesothelium and blood vessels was almost completely suppressed. (4) Results were interpreted from the standpoint of the double-potential theory (Yamada, 1947).