Somatic Myogenic Action in Embryos of Fundulus Heteroclitus

Abstract

Five phases in the development of motility in this species can be recognized. (A) Localized spontaneous contractions within the trunk (part attached to the yolk-sac) only; tail-bud, immotile. (B) All movements spontaneous; integrated contractions throughout the trunk, but slow in execution and relaxation; localized contractions only in the proximal part of the tail; distal part, immotile. (C) The general integrated contractions are quick; involving about the proximal half of the tail with the trunk; localized contractions only in the most distal myotomes of the region involved in the general integrated contractions, or just behind it, the more distal myotomes being immotile. This phase is marked by the beginning of tactile sensitivity. (D) The general contractions involve the entire trunk and tail; localized spontaneous contraction only at the tip of the tail and in the pectoral fin. (E) Localized movement of the tail has ceased, and the pectoral fin, like the caudal fin, becomes integrated with the total action pattern, without capability of local reflexes.

The skin of the embryo and yolk-sac being highly impervious to curare, the drug was injected into the yolk-sac, or the latter was cut open while immersed in the solution in order to distinguish myogenic from neurogenic action. In embryos of phase A, under the action of curare, contraction of the myotomes occurred as much as 15 minutes after the circulation of the blood had ceased. This is accepted as conclusive evidence that all muscular contraction of this phase is purely myogenic. In embryos of phase B, injected with curare, spontaneous movements are localized at different levels within the trunk and originally motile part of the tail: integration of the system as a whole is lost.