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A B S T R A C TSensory Rohon-Beard cells of R a n a pipiens embryos were examined for dependency on the peripheral innervation. Flank ectodesm was surgically removed, the area being maintained skinless by lateral parabiosis. The RohonBeard cells in each side of the spinal cord were counted at a later developmental stage; the number was always less on the side with the reduced skin area. Intact animals, operated controls, and tails of the parabiotic pairs did not show such a bias i n cell distribution. Deprivation of the neurotrophic periphery is postulated as the cause of the premature disappearance of the Rohon-Beard cells; it is further suggested that the disappearance of these cells during normal development might be effected through the peripheral innervation.The trophic relationship between the nervous system and its peripheral innervation (reviewed by Jacobson, '70) is of two basic types: the maintenance of the peripheral organs (e.g., taste buds, muscle) by innervation and conversely, the maintenance of neurons by the periphery (e.g., lateral motor columns, optic tectum). The present study focuses on the maintenance of sensory neurons which disappear in the course of amphibian development. These sensory neurons, the Rohon-Beard cells, are located within the spinal cord, and they innervate the skin and musculature of the embryo and young larva (reviewed by Hughes, '57). This sensory system is functionally replaced at later stages by the spinal ganglia in many vertebrate species. The causal factors behind the disappearance of the Rohon-Beard cells are unknown, the two major possibilities being hormone level and peripheral dependency. The hormonal aspect has been considered for Rana pipiens by Stephens ('65) and for Eleutherodactylus ricordii by Hughes ('66). That Rohon-Beard cells might respond to indirect changes in the periphery has been suggested by Stephens ('64). Prior to inquiring into peripheral dependency and the means by which the RohonBeard cells might be deprived of their periphery in the normal course of development, it is first necessary to establish that there is a detectable dependency on the periphery. This was accomplished by surgically removing lateral ectoderm from embryos and maintaining the skinless area J. EXP. ZOOL., 1 8 5 . 209-216. by parabiosis of two operated embryos side to side. MATERIAL AND METHODSRana pipiens embryos were obtained by the standard induced ovulation technique of Rugh ('34). Some of the embryos of the single clutch were slowed in development by transient cooling at 15°C. The operations of parabiosis and of sham surgery were performed during a three day period on embryos of stages 15 to early 17 (Arabic numerals represent the embryonic stages of Shumway, '40; Roman numerals represent the larval stages of Taylor and Kollros, '46). The surgery was done in Holtfreter's solution following the procedure of Hamburger ('60). With glass needles and glass rods, a large patch of ectoderm lateral to the nephric ridge along the length of the flank was circumscribed and remo...

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