For many years research has been carried out on the problem of the cellular response in the developing nervous system to the presence or absence of peripheral stimuli. Detwiler,1 working with amphibian larvae, demonstrated the hyperplasia of sensory ganglions when they were connected with a transplanted limb. That the central nervous system responds to increased peripheral stimuli by an excessive proliferation of cellular elements has been demonstrated clearly by Burr,2 who showed that there is a definite hyperplasia in the pars dorsalis thalami and in the cerebral hemisphere when an additional nasal placode is grafted adjacent to the normal one in Amblystoma. Further, Hamburger3 showed that the motor horn areas of the cord underwent a hyperplasia when limb primordia were grafted in chick embryos. The opposite procedure, removal of an end organ, has been shown to bring about a hypoplasia in the central nervous system, in the