CHOLINESTERASES AND MOTOR END-PLATES IN DEVELOPING DUCK SKELETAL MUSCLE

End-plates in the thigh muscles of duck embryos were first recognized with myristoylcholine as substrate at the 19th day of incubation. Each appeared as a cholinesterase-positive dot surrounded by a small halo which rapidly increased in size during the 20th and 21st days. The endplates were usually oval in shape, averaging 33 µ x 25 µ with a subneural apparatus 5-12µ wide. The latter contained refringent lamellas arranged transversely in a palisade fashion. From the 21st day to the day of hatching (27-29 days) the number of end-plates progressively increased. After hatching, the myristoylcholine-reacting end-plates were difficult to locate. With acetylthiocholine as substrate, the embryonal end-plates were not demonstrable; however, the posthatched tissues showed numerous end-plates. The nerve trunks and nerve fibers gave a faintly positive myristoylcholine reactions in all stages after the 19th day of incubation. On the basis of the effects of eserine and diisopropyl fluorophosphate, the structures reacting with myristoylcholine and acetylthiocholine contained specific chohinesterase. The end-plates containing nonspecific cholinesterase also appeared on the 19th day of incubation and appeared to increase gradually in number until the 15th postembryonic day; thereafter they seemed to decrease.