Abstract
The processes of isolated rat sympathetic neurons growing in culture were marked with glass or carmine particles and observed with timelapse microphotography. Particles on the processes moved with the cell in relation to the dish and underwent continual small jerky movements. They did not, however, show any over-all distal motion and for long periods, during which the growth cone progressed more than 100 mu, the particles remained at about the same distance from the cell body. The most obvious explanation for this result is that new fiber surface, and perhaps the plasma membrane, is deposited in the region of the growing tip.