Sixty-nine specimen 's were incubated for 24 hours, and had usuany reached the late primitive streak or head process stage before injection. Eight of these we're fixed after either I or 2 hours' treatment, and the remainder after about 24 hours.Twenty-four eggs were incubated for 48 hours before being injected ; a young embryo had formed by this time in each egg and it possessed a well formed brain, a few pairs of somites and a beating heart ; eight of these specimens were fixed after 24 hours' treatment, whilst the remainder were not examined until 3 days after injection.Three batches of amphibian eggs were, used, two being Rana temporaria, the other Bufo bufo. Every batch was divided into five equal groups, each consisting of about 50 eggs complete with membranes and jelly. Four groups were placed