Origin of Regenerating Brain in the Earthworm

Abstract

Hescheler described in detail the histogenetic processes following decapitation of the earthworm. He stated that the cerebral ganglia are formed from cells of the body wall ectoderm which have migrated posteriorly and medially. Nuzum and Rand showed that this method of cerebral ganglia regeneration is by no means constant in Allolobophora foetida and that under certain conditions the epithelium of the dorsal pharynx wall may also contribute to brain formation. The latter had dissected only the cerebral ganglia from the head leaving the pharynx wall uninjured, whereas Hesche-ler's description was based on removal of 5 head segments. In the course of a study of regenerating earthworm heads from another viewpoint, evidence confirmatory of Nuzum and Rand's findings was uncovered. In the present work 5 head segments were removed from Helodrilus caliginosus and in some cases 10 segments of ventral nerve cord in addition. In most specimens cerebral ganglia regeneration followed closely the course described by Hescheler. In several cases, however, there was an obvious contribution from both body wall ectoderm and dorsal pharynx wall to the regenerating brain. In one striking instance (Fig. 1) only cells from the epithelium of the dorsal pharynx wall were passing to the regenerating ganglia and no evidence of movement of cells from the dorsal ectoderm was anywhere apparent. In this specimen there was evident a stream of epithelial cells passing dorsally and laterally, in a spiral manner, from the dorsal pharynx wall epithelium towards the position occupied by the new brain. The muscle and connective tissue layers of the pharynx wall have not regenerated and a basement membrane is lacking in this region.