Studies in Embryonic and Larval Development in Amphibia : I. The Embryology of Eleutherodactylus ricordii, with Special Reference to the Spinal Cord

Abstract

In the West Indian Anuran genus Eleutherodactylus the larval stage is wholly suppressed. As in other systems of organs there are features in the spinal cord which are correlated with this embryonic type of development. The cord in the trunk region resembles more that of a chick embryo than that of a larval Amphibian. Within the thick neuroepithelium mitoses in the ependymal zone persist much longer than in larval forms. There is a relatively thin layer of white matter. The Rohon-Beard system of sensory neurones persists, but does not precede the development of the dorsal root ganglia. In the trunk there are no conspicuous primary motor-cells as in larval Amphibia. The numbers of cells within the dorsal root ganglia and ventral motor horns have been counted both in the embryo and in the adult. From the first, the numbers of cells present in both are similar to those in the adult. During development, increase by mitosis is opposed by cell degeneration. Pycnotic nuclei are conspicuous in both spinal ganglia and ventral horns in the embryo. As soon as the spinal ganglia are recognizable, their relative sizes are similar to those of the adult. The ganglia at limb levels are already larger than those elsewhere a day before nerve fibres first enter the limb-buds.