The distribution of motoneurones supplying chick hind limb muscles.

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Abstract
The motor nuclei supplying many of the hind limb muscles were localized in late chick embryos (stage 36-37; 10-11 days) by utilizing the technique of retrograde transport of horseradish peroxidase. Each nucleus was localized in a characteristic position in both the rostro-caudal and transverse plane of the spinal cord with only slight individual variation. Each motor nucleus consisted of an elongate, coherent cluster of labeled cells, with few cells occurring outside the cluster. There did not appear to be extensive overlap of nuclei nor extensive intermingling of motoneurons projecting to different muscles. The position of a motor nucleus in the transverse plane was not correlated with whether its muscle was used as an extensor or flexor. Nor were adjacent nuclei necessarily co-activated during normal unrestrained walking movements as deduced from e.m.g. [electromyogram] recordings. The position of a motor nucleus was not correlated in a topographical manner with the adult position in the limb of the muscle to which it projected. While no correlation was found between the rostrocaudal position of a motor nucleus and the embryonic muscle mass from which its muscle was derived, such a relationship existed for the medio-lateral position. All muscles arising from the dorsal muscle mass, regardless of their function or adult position, were innervated by laterally situated motoneurons and all muscles arising from the ventral muscle mass by medially situated motoneurones. Motoneuron position is apparently most closely correlated with ontogenetic events presumably involved in the matching of the motoneuron with the correct muscle in the periphery. The central connections onto motoneurons responsible for their proper activation apparently cannot be achieved by a simple mechanism based largely on the position of the motoneuron soma.