The origin of the segmental musculature of the tail of the Axolotl (Ambystoma)

Abstract

Summary. By means of vital stains the rudiments of the tail structures present in the posterior one‐fifth of the medullary plate were delineated and the extent of the chordal and somitic mesoderm remapped. The cells that will form the myotomes of the posterior part of the trunk and the whole of the tail are present in the posterior medullary plate at stage 15. By stage 16–17 the trunk material is invaginated. The tail mesoderm is enclosed by the closure of the neural folds. The neural folds of the posterior two‐fifths of the medullary plate are responsible for the formation of the dorsal and ventral tail fin (see 11 below). Perichordal and other mesenchyme of the tail has its origin in the posterior medullary plate. The lateral lips of the blastopore take part in the formation of the proctodaeum which produces sprouts growing forwards to meet the descending pronephric ducts. By operative techniques (transplants, deficiency experiments, etc.) the degree of organization and the inductive powers of the posterior medullary plate were determined. At stage 15 the posterior fifth of the neural plate is fully determined to form segmental muscular tissue. The establishment of its antero‐posterior axis, however, depends on the presence below it of the axial structures, the neural tube and notochord. Extirpation of the posterior medullary plate results in deficiencies in somitic mesoderm of the tail and hind end of the trunk. Mesenchyme is present and is, no doubt, the product of the neural crest. Medullary plate, whether its prospective fate be neural or muscular, possesses the power to form a tubular structure without the assistance of the neural folds. In transplants the segmental tissue formed by the posterior medullary plate closely resembles neural tube cytologically, relating tail muscle to its origin in the medullary plate. The neural fold of one side of the medullary plate is capable of forming a bilaterally symmetrical fin. The presumptive rudiments of the tail possess remarkable powers of stretching.