We owe to Leydig (1) the first demonstration that a secretory process is a possibility for the skin of the Fish. In some twelve genera he described the occurrence of specialized cells in the epidermis, to which he gave the common name of “Schleim-zellen,” though subsequent research has shown that several varieties of structure were included in this generic term. Kölliker (2) in 1860, in an investigation of the skins of Myxine and Petromyzon , differentiated two cell-forms. Observing the “thread bodies” of the mucous sacs of Myxine , first clearly described by Johannes Müller (3), he recognized their cellular nature, and found that the epidermis held similar, though less complex cells, from whose contents also, by appropriate treatment, a thread could be obtained. To these epidermic cells of Myxine the name “Fadenzellen” was applied. In addition, clear or finely granular cells, termed “Schleimzellen,” were described.