ABSTRACT
If a young Urodele gastrula is cut or constricted into two along the frontal plane, so that the dorsal and ventral halves are separated, the dorsal part contains the whole organization centre but an abnormally small amount of presumptive ectoderm; nevertheless, as Spemann (1903) and Ruud & Spemann (1922) showed, it regulates to form a complete embryo of normal proportions but of small absolute size. Schmidt (1933) demonstrated the same thing for Anuran gastrulae. Thus an absence of ectoderm causes modification of the normal development of the organization centre. We have much less information of the effect of increase in the proportion of presumptive ectoderm. Bruns (1931) added fairly small pieces of ectoderm to entire gastrulae, and obtained embryos apparently indistinguishable from normal ; but the amounts added were so small that one could not expect easily visible effects. In the experiments to be reported here, very much larger quantities of ectoderm were added to normal gastrulae, and, as will be shown, it was found that the presence of this added material caused an increase in size of the invaginated roof of the gut, so that an embryo of normal proportions but increased size was developed.
