Neuronal Differentiation in Implanted Foetal Cortical Tissue

THE capacity for continued growth and differentiation, up to a certain point, of grafted fragments of the embryonic nervous system has been amply demon- strated in lower vertebrates. In the chick, for example, it has been shown that if segments of the spinal cord anlage are implanted on the allantois, the cells of the neural plate show high powers of proliferation and typical nerve-cells with abundant processes are formed (Danchakoff and Agassiz, 1924). In the cord the three layers-ependymal, mantle, and marginal-are developed, and the neuroblasts of the mantle layer become arranged in groups resembling the ventral and dorsal horns of grey matter in the normal cord. Similar powers of differentiation were found by Hoadley (1924) in portions of the embryonic mesencephalon of chicks of 48 hours' incubation, which had been grafted on the chorio-allantoic membrane. The roof of the mesencephalon acquired a laminated structure similar to that which is characteristic of the tectum opticum of a normal 8-day chick, with outer and inner cell layers and outer and inner fibre layers.