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Studies in Embryonic and Larval Development in Amphibia: II. The Spinal Motor-Root

1959; The Company of Biologists; Volume: 7; Issue: 2 Linguagem: Inglês

10.1242/dev.7.2.128

1477-9129

Arthur Hughes,

Developmental Biology and Gene Regulation

ABSTRACT The development of the spinal motor-roots is described in the Anurans Xenopus and Eleutherodactylus, together with observations on the embryonic roots of Bufo, the lizard Sphaerodactylus, and the chick. From a primary motor-cell in the embryo of Xenopus an axon process grows along the surface of the cord and finally turns outwards to traverse a myocommatal plane, thereby forming a ventral root. As in other larval Amphibia, the longitudinal motor-tract and the ventral roots originate together, though not necessarily by identical methods in all. In the recently hatched larva, movement is inhibited by transection of the central nervous system behind the otocyst. At these levels there must be a centre for the co-ordination of swimming movements. In mid-larval stages the ventral roots of Xenopus and Bufo arise from the longitudinal motor-tract of the cord; cell-body and myocommatal terminal of the axon are longitudinally displaced, though the precise arrangement varies at different levels in the cord. The ventral roots of the trunk of Eleutherodactylus are wholly separate and distinct from the longitudinal motor-tract, the fibres of which they cross at right angles. The arrangement of the ventral roots of the tail resembles that in other Anura. In embryos of the lizard Sphaerodactylus the ventral-root fibres arise directly from cells of the basal plate of the cord and run at various angles through the white matter to emerge as spinal motor-roots. In the chick embryo ventral-root fibres run directly at right angles through the white matter. On the basis of observations on the behaviour of nerve fibres in tissue culture, it is suggested that there is a general correlation between the angle of approach of two growing fibres and their chances of permanent anastomosis.