Projections of individual axons in lamprey spinal cord determined by tracings through serial sections

The brain and spinal cord of one adult sea lamprey, Petromyzon marinus, 12 cm in length, were fixed in glutaraldehyde and osmium, stained in toluidine blue, embedded in Araldite, and sectioned serially at 5 μ. Over 50 large axons were traced through the spinal cord to their probable cells of origin. The axon of one giant interneuron was followed from the posterior spinal cord rostrally into the lateral medulla of the brain. A formerly unrecognized pair of Müller axons was found to originate from the reticular cells I2. An unusual feature of some Müller axons was their migration out of the ventral columns and their subsequent branching.