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 THE REGENERATION OF LOST PARTS IX ANIMALS. 41 / tentacles, and the other characters of a ' head ' ; and fission only then takes place, when the tvo halves are ready to begin inde- pendent existence in a comparatively perfect state. In Arthropods and Vertebrates the capacity for regeneration becomes restricted to the less important organs, such as limbs or tail. M. Leon Fredericq, 1 has lately given us an accurate account of the way in which crabs or lobsters voluntarily relinquish their limbs in self-defence, just as a rat is said to bite off its paw to escape from a trap. And a lizard leaves its tail in our fingers, knowing well that it will soon get another. The perfect regene- ration of an amputated limb in newts is another well-known case. So far the facts of this recapitulation are for the most part well-known and familiar, but our knowledge of them has lately been advanced in two directions. We have acquired important new facts, first as regards the histological processes of regene- ration, and secondly as regards the precise characters of some regenerate organs. The crucial point in the former half of the subject is the resem- blance of the processes of regeneration to the original phenomena of embryonic development : in the latter, the important new fact is the occasional indication of reversion to a primitive type in the new-formed organ. Billow has lately studied the histological structure of the growing tip of the tail in Lumbriculus -i-arwiatus. The hindmost of a series of transverse sections made through the anal extremity of the worm showed two cell-layers only, the ectoderm and endoderm : sections slightly anterior to these showed the development of a mesoderm, by proliferation from the point of junction of endoderm and ectoderm, exactly as in its embryonic origin. 2 The nerve-ganglia or commissures of the central chain are developed from the ectodermal layer : the gut from endoderm ; the muscle-plates, segmental organs, liver- cells and vascular system, from the mesoderm. The ambulatory bristles are formed from ectoderm, but their musculature is mesodermic. The whole process is conspicuously similar to or identical with the development of the embryo, and Biilow even talks of his posterior sections, with their endoderm and ectoderm, as constituting a two-layered ' gastrula '. It is not that the organs already formed are growing bigger or multiplying by sub- division, but, from the origin of the mesoblast to the construction of complex organs, all is going on upon an embryonic plan. Biilow extends this result to the case of regeneration. In a divided worm, he says, the tail is regenerated from cell-layers developed in the same way and exactly equivalent to the three layers of the embryo. 1 Arch, de Zool. expJr., (2) i., p. 413, 1883. 2 Zeitschr. f. wiss. Zool., loc. cit.