Thermoregulation and Mate - Selection in Fowler ' s Toads ?

undifferentiated periphery and immaturity of hindlimb motoneurons. The early appearance of the pattern generator for coordination between the hindlimbs, activation of this pattern generator only during operation of the locomotor program for tail oscillations, and the rigid phase and frequency locking between bursts of primary and hindlimb motoneurons all suggest that the two pattern generators may be identical or share common neuronal elements. However, the experiments are only suggestive and do not rule out the possibility that the tail and developing hindlimbs are coordinated by separate, discrete mechanisms that are phase coupled and activated by a common source. In either case, the late-developing bilaterally synchronous bursting of the hindlimb motoneurons probably represents a modification or addition to the mechanisms coordinating alternate stepping. A behavioral parallel to the sequence of electrophysiological activity described here is found in the successive development of axial muscle flexions during swimming, alternate stepping, and then frog kicks (8). The sequential appearance of lateral trunk flexions, lateral trunk flexions accompanied by limb movements, and limb movements without lateral trunk flexions has been amply documented in humans and other species (9, 15). Thus, the results of further experiments with this preparation may provide insights into the organization and early development of locomotion in other vertebrate species. DONALD J. STEHOUWER* PAUL B. FARELt Department of Physiology, School of Medicine, University of North Carolina, Chapel Hill 27514