Responses from motoneurons were recorded with microelectrodes, from the spinal cords of kitten fetuses and newborn kittens between 40 days' gestation and a few days after birth. As in the adult animal, intracellularly recorded action potentials by either ortho- or antidromic shocks have two components, "A" and "B" or IS and SD. The action potentials of the adult and immature motoneuron differ mainly in the afterpotentials which are absent in the fetal cell in "good" condition. Repeated stimulation or deterioration of the cell resulted, however, in the appearance of depolarizing and hyperpolarizing afterpotentials. No major differences were found in the mode of anti- or orthodromic invasion of the adult and fetal motoneuron, but the degree of invasion of the soma-dendritic complex may be somewhat less in the fetal cells. The ventral root discharge by dorsal root stimulation could be obtained in the fetus 3 weeks before birth. This reflex discharge was concluded to be monosynaptic. Excitatory postsynaptic potentials, probably monosynaptically activated, could be recorded from inside motoneurons by stimulation of dorsal root or peripheral nerves. The most remarkable change during prenatal development was an increase in the speed and efficacy of the excitatory synaptic potentials which showed a marked change during the last weeks of prenatal life.
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