418 D'ARCY w. THOMPSON: Fraisse l has in like manner shown that, in the regenerating tail of a Urodele Amphibian, the histological processes are exactly comparable to those of embryonic development ; though we cannot talk of a ' gastrula ' in this case, as the faculty for regeneration does not extend to the cloaca. (Of course, as Fraisse points out, the details of the process of regeneration are in many cases somewhat modified and have their embryonic features marked by altered circumstances.) Semper 2 had noted, some years before, the affinity between the phenomena of budding in Nais and the process of embryonic development ; and both he and Billow have traced an intimate likeness between the processes of budding and regeneration of organs. These observations have, I think, a philosophical even more than a morphological importance, but to that point I shall return. Eeversionary characters in regenerate organs have been de- scribed in the case of certain Crustacea by Fritz Miiller. 3 For instance in Atijoida Protomirum, a species of Prawn, a mutilated chela is sometimes replaced by one not of the form characteristic of the species, but resembling that found in Carodina, an allied genus. The claw in Carodina is simpler and apparently more primitive than in Atyoida, for the bristles that it bears are short and small, and it is not hollowed out and spoon- shaped as in the latter. In the same crustacean, the fifth pair of feet are peculiar, and are distinguished from the adjacent ones by the want of cer- tain movable bristles, and by the presence of a brush of hair on the last joint. But when amputated, these feet are restored on the normal pattern of the others, though in course of time after a succession of moults they recover their own distinctive charac- ters. Very similar, if somewhat less striking, results have been ob- tained by Fraisse, 4 from experiments on lizards. He finds that, especially in Lacerta muralis and in Geckos, but also in our com- mon Lacerta agilis, the regenerate tail is commonly darker in colour than its predecessor. And transverse sections show that this darker tint is not due merely to the transparency of the young epidermis rendering the dermal pigment more plainly visible, but that the pigment encroaches upon the epidermis, occupying the interstices between its cells, so that the dermal chromatophores are well-nigh hidden. And the epidermis is found also to contain pigment-cells, such as occur in the embryo, but not normally in the adult. In course of time the pigment vanishes from the epidermis, and the lizard regains its proper hue. The 1 Biol. Centralblatt, iii., No. 20, p. 821, Dec. 15, 1883. 2 " Verwandtscliaftsbeziehungen tier nieclern Thiere," iii. Arb. aus d. Inst. Zool. Wurzburg, iii. 3 Kosmos, viii., p. 388. 4 Biul. Centralblatt, lo'c. cit.
