CAVANAGH1, using the technique described by Nakai2, has filmed the growth of isolated cells from chick-embryo dorsal root ganglia. By the application of a similar technique, growth of new processes has been shown in isolated cells from dorsal root ganglia of new-born rats up to 7 days old. The Wag-strain rats were decapitated, the spinal cord separated, and the ganglia teased out with needles under microscopic vision. The ganglia were torn open to release a large number of cells into the culture Medium 199 (ref. 3) in well slides. The slides were covered with cover-slips, sealed with petroleum jelly, and incubated at 37° C, They were examined and photographed under phase-contrast microscopy immediately on incubation (20 preparations), and then three times a week; in this series, cultures from 15 young rats, and 11 adult rats, were observed between 7 and 14 days after the beginning of incubation. No processes were visible in the freshly incubated cells, nor were they in the cells from the adult ganglia (Fig. 1). However, in 13 out of 15 preparations of cells from young rats, straight or angular processes of length equivalent to from one-quarter to two times the cell body diameter could be seen in about a quarter to a third of the cells in each preparation (Fig. 2). Growth cones could sometimes be seen either at the tips of the processes or where they changed direction.