Experiments upon the Segmentation of Spinal Nerves in Salamander Embryos

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Abstract
The relation between segmentation in the spinal cord with that in the mesodermal somites was tested by 3 sets of expts. (1) Somites 2, 3, 4, and 5 were excised in Amblystoma embryos in Harrison''s stages 25-27. Limbs developed in all surviving embryos; 50% of the limbs were normal, with normal function. The dorsal root ganglia failed to segment normally, and both sensory and motor nerve roots were variable in no. (2) Somites 3, 4 and 5 were removed on the right side, and in the wound were grafted 4 or 5 smaller somites from a more caudal region of another embryo. The % of normal limbs with normal function was high. Of 10 cases examined, 8 showed an extra ganglion, as well as an additional spinal nerve with both sensory and motor roots. (3) Neural crest cells were destroyed by making an incision between the spinal cord and the somites. All cases examined later showed the normal no. of spinal ganglia with the typical arrangement. It is concluded that segmentation of spinal ganglia and nerves is determined by mesodermal metamerism, and that there is no support of the generally held view that nervous metamerism is intrinsic.

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