BREVIORA MniseiuitM of Comparative Zoology Cambridge, Mass. June 17, 1959 Number 111 ON THE CAUDAL NEUROSECKETORY SYSTEM OF THE TELEOST PISH, FVNDIJLVS HETEROCLITUS L. By Uno Holmgren ^ Zoologifiil Institute, Uppsala, Sweden Biological Laboratories, Harvard University INTRODT'CTION It Avas early observed that a bulb-like structure was associated with the terminal portion of the spinal cord of certain fishes (AVeber 1S27, Rauber 1877, Arsaky 1813, Ziehen 1903, and P'avaro 1925). This outgrowth of the spinal cord of different systematical g-roups of teleosts was described by Favaro (1925), who studied a large number of species. Because of the histologi-cal and morphological resemblance of this organ to the neuro-hypophysis, he named it "Ipofisi caudale." Secretory cells in the terminal portion of the spinal cord were described first by Dahlgren (1914), and later by Speidel (1919, 1922). Tlie latter also gave a histological description of some "irregular glandular cells," sometimes called " Dahlgren 's cells," in the terminal portion of the spinal cord of certain elasmobranchs and teleosts. Recently Enami (1955 a) investi-gated the caudal portion of the spinal cord of the eel, Anguilla japonica, with respect to the secretory activity of " Dahlgren 's cells" and found that neurosecretory cells were present in the terminal portion of the spinal cord. They extended caudally from the level of the last sixth or seventh vertebra. These neuro-secretory cells, according to Enami, resembled the secretory cells of the hypothalamus, and had axons which served as neurosecre-1-FflldW (if the t 'oiiniioinvcaltli l'''ini(1.