Abstract
1. The presence of neuroid conduction in the skin ofXenopus laevis embryos gives them a precocious capacity to respond to stimuli. This conduction system constitutes a precursor of the sensory system, in that interactive electrical coupling exists between the myotomes and the epithelium prior to the appearance of sensory nerves. 2. Propagated electrical activity in the epithelium ofXenopus laevis embryos was recorded using extracellular electrodes for morphological stages in which myotomes are presumably not innervated (i.e., up to Nieuwkoop and Faber, stage 27). Relative to the external medium, transepithelial negativegoing spikes are superimposed on a positive transepithelial potential. Only epithelial activity was recorded; myotome activity was not recorded directly. 3. Specific correlates of transepithelial electrical activity were identified for the characteristic maneuvers for four behavioral stages. These stages are designated nonmotile, premotile, head flexure and coil, and alternating flexure. Earliest epithelial electrical activity in the nonmotile period, is thought to correspond with cytoplasmic contractions in myotomes. During the premotile period, which corresponds with Nieuwkoop and Faber stages 22 to 24 at a rearing temperature of 22° C, movement is minimal and only to one side. Head flexure and coil involve most of the myotomes capable of contractions. This activity usually involves a single contraction. Next the animal spontaneously bends from side to side repetitively during the alternating flexure stage. This corresponds with stages 25 to 27. In all stages different behavioral maneuvers can be identified from the electrical records. 4. Behavioral maneuvers were studied from film records to correlate movements with electrical recordings. 5. Morphological and behavioral stages were correlated at three rearing temperatures. The appearance of behavioral stages are retarded more than the appearance of morphological stages at reduced temperatures.
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