Analysis of the development of the nervous system of the zebrafish, Brachydanio rerio: I. The normal morphology and development of the spinal cord and ganglia of the zebrafish

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Abstract
In embryos and young fry of the zebrafish, Brachydanio rerio, the sensory function of the nervous system is fulfilled by a primitive transitory system of Rohon-Beard cells. These are replaced 2 or 3 weeks after hatching by the developing spinal ganglia. The spinal ganglia are typically tear-drop-shaped and occupy a dorsolateral position relative to the spinal cord, with the exception of the first three pairs of ganglia which are large, round, and lie ventrolateral to the cord. As the fish grow, the spinal ganglia grow in size, due to an increase in cell size, and an increase in cell number. This is different from the situation in mammals and chicks, for example, in which the number of cells reached in embryonic stages remains constant, and growth of ganglia is achieved only by means of cell enlargement.