Cell migration into the “established” lateral motor column in Rana pipiens larvae: I. Brachial spinal cord

It has previously been assumed that once the lateral motor columns are set aside as a definitive region of the spinal cord in Rana pipiens larvae (about stage V of Taylor and Kollros, '46), no further movement of cells into the column occurs. Since the cells of the lateral motor column (LMC) arise from cells dividing in the ventricular zone of the spinal cord, it is possible to label some of the nuclei of cells destined for the LMC with thymidine‐H 3 . It was, thus, shown directly that cells enter the brachial LMC at stages after its initial formation. Larvae of R. pipiens were injected with thymidine‐H 3 at various stages of development, and subsequent autoradiographs were made of serially sectioned spinal cords. Animals injected through stage VII exhibited a greater frequency of labeled LMC cells at later stages than did those injected after stage VII. By the onset of metamorphic climax, stage XIX, no further production of cells for the LMC could be detected. It is suggested that cell turnover in the developing LMC may have a role in the formation of the proper neuro‐muscular and neuro‐neuronal connections.