The different centers of the C. N. S. develop at an unequal rate. Barcroft and Barron recently studied this development on the movements of sheep fetuses. If the high cone. of choline esterase present at synapses of the C. N. S. has a physiological significance it should be expected that a relationship exists between the time of appearance of the high enzymatic cone. during growth and the time of beginning function of the centers. Observations on the enzyme cone. in spinal cord and brain of sheep fetuses indicate that such a relationship does exist. In the spinal cord, which controls the movements at an early age, the enzyme appears in high cone. at that age, whereas it is still low in the brain centers. There it rises steeply in the last wks. before birth.