More than a century ago, Santiago Ramon y Cajal examined the
cellular morphology of developing neurons and described how axon
tips extended and sought out places of connection with other neu-
rons. From his examination of static microscopic images of growing
neurons in chicken embryos, Ramon y Cajal hypothesized that the
tips of growing axons possessed exquisite mechanisms of sensitivity,
motility, and guidance. He writes in his autobiography:
In my inmost heart, I regard as the best of my work at that time the ob-
servations devoted to neurogeny, that is to the embryonic development
of the nervous system. ...I had the good fortune to behold for the first
time that fantastic ending of the growing axon. In my sections of the
three-days chick embryo, this ending appeared as a concentration of pro-
toplasm of conical form, endowed with amoeboid movements. It could
be compared to a living battering ram, soft and flexible, which advances,
pushing aside mechanically the obstacles which it finds in its way, until
it reaches the area of its peripheral distribution. This curious terminal
club, I christened the growth cone.