Microchemical procedures were employed for the quantitative assay of acetylcholinesterase ( AChE) and choline acetyltransferase (ChAc) activity in the brachial spinal cord of chick embryos from 3.5 to 21 days' incubation. The activity of AChE increased nearly 10-fold during the 60-hour period from stage 22 (3.5 days) to stage 30 (6.0 days). The level of AChE at six days was essentially the same as that at hatching. During the intervening period, the level of AChE activity remained unchanged with the exception of minor transient peaks a t days 7, 13, and 18. The activity of ChAc increased 22-fold during the developmental period studied. The ontogenetic pattern was characterized by two periods of rapid increase in acitvity. From stage 23 (3.75 days) to stage 31 (7.0 days), ChAc activity increased approximately 5.4-fold and from stage 38+ (12.5 days) to stage 44 (18 days) ChAc activity increased approximately 3.6-fold with the major increase occurring during the 48-hour period from stage 42 to stage 44. The ontogenetic patterns for both enzymes are discussed in relation to morphological and functional differentiation. It is suggested that ChAc activity is a more reliable index of functional differentiation than is AChE and that the initial increase in AChE activity may be associated with some function other than synaptic transmission.
The enzymes associated with cholinergic transmission, AChE and ChAc, increase in specific activity during the development of the central nervous system in amphibians (Sawyer, '43;