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The two elementary effectors, cilia and muscles, are found throughout almost the whole range of the animal kingdom. In such simple organisms as sea-anemones these two effectors are about equally developed and are usually mutually concerned in many of the common activities of these animals, such as the production of the respiratory currents, the appropriation of food, and the like. Vertebrates, on the other hand, with their growth in bulk have in large measure discarded cilia and have developed muscles as the almost exclusive means of response. Nevertheless, in parts of their bodies such as the reproductive system, both kinds of effectors, cilia and muscles, are mutually active and co-operate in carrying out certain functions of this important system. No one can observe the active oviduct of a vertebrate without being impressed by its striking resemblance to the responding coelenterate body. Peristalsis, antiperistalsis, and other co-ordinated muscular movements combined with ciliary activity, are the elements on which both the organs of coelenterates and the oviducts of vertebrates appear to rely for successful operation. The oviducts possess the remarkable property of conducting the reproductive elements in two opposite directions; sperms are conveyed by them from the exterior to the neighbourhood of the ovary and ova, or embryos are transported in the reverse direction. These two processes of inward and outward conduction are not unlike the swallowing of food by the gullet of the sea-anemone and the discharge of excrement by the same organ. Are the operations of the vertebrate oviduct performed in the same way as those of the gullet of the coelenterate or not? In other words, do the oviducts exhibit muscular movements associated with normal or reversed ciliary beat to accomplish the appropriate transfer of the vertebrate genital products, or are these responses in vertebrates based on entirely different principles? These and other related problems are discussed in the following pages.

VII. The passage of sperms and of eggs through the oviducts in terrestrial vertebrates