1. A method for the study of the nervous system of vertebrate embryos by methylene blue vital staining is described. A reliable technique for rendering the preparations permanent is described.
2. An adaptation of the silver ‘on-the-slide’ method is given.
3. Three types of sensory intramedullary neuron are described in the spinal cord of recently hatched ammocoetes, or prides, of Lampetra planeri. All three are regarded as types of Rohon-Beard cell.
4. Four contemporary correlating types of cell are described in the cord: large internuncial neurons with a dendritic system which reaches the contralateral dorsal funiculus; cells of the Commissura Infima; oblique fibres, descending caudally from the sensory to the motor tracts; and small internuncial neurons with short dendrites.
5. The relations of the Miiller fibres in the trunk are described in part.
6. Two types of motor neuron have been found; the more fully developed corresponds to the primary motor neuron of aquatic larvae of other anamniote vertebrates.
7. The peripheral fibres of the somatic system of the trunk are described.
8. The neurological pattern revealed is compared with that in adult Lampetra: the divergences from the vertebrate pattern found in the cord of the adult are not found in the young ammocoete, which in this, as in so many respects, is a good prototype of gnathostome vertebrates.
9. The probable functional pattern is compared with that found in a similar stage of Amblystoma.
Neurons of the correlating and motor system appear not to have been described before in ammocoetes less than 1 year old.