[93] This seems to be true for urodeles, but whether it is true for the anurans is rather a question of definition, as I have pointed out in my book on _The Development of the Frog’s Egg_. [94] The attachments of the muscles may be the cause of the break in the middle of the vertebræ, rather than between two vertebræ. [95] _Prodromo_, 1768. [96] Philipeaux, _Comptes rendus de l’Acad. des sciences de l’Institut de France_, Année 1866, 1867. [97] Todd (_Quarterly Journal of Science, Literature, and Arts_, Vol. XVI), Blumenbach, Treviranus, Von Siebold. [98] How the tentacles could have gotten into their normal position is not explained. [99] The foot sometimes pushes out through one of the slits made by the bristle instead of out of the mouth. [100] I have given elsewhere (_The International Monthly_, March, 1901) a fuller treatment of the gastræa theory from the historical point of view. [101] It may be pointed out that there may be really several kinds of homology, such as homology due to similar origin of the blastomeres, or to their position, or to their fate, etc. The confusion that has arisen may in part result from the attempt to make homologous parts agree in all points. [102] That is, one not depending on inheritance through adult forms. [103] _Biologisches Centralblatt_, XV, ’95. [104] A small amount of embryonic mesenchyme may come from some of the ectodermal quartettes of the embryo and produce the branching muscles of the head, but not the characteristic muscles of the trunk. [105] Cosmos, Vol. VII, p. 388. [106] King pointed out the fallacy of this argument. [107] Roux’s earlier experiments in 1885, in which the unsegmented or segmented egg was stuck and a part of its contents removed, the remaining part making a whole embryo, will be considered in another connection. [108] This had been first discovered by Newport in 1851. [109] The cross-section _C_ is reversed as compared with the half-embryo _B_. [110] This difference is due, I suppose, to the amount of injury that the nucleus of the injured half may have suffered. [111] The development of isolated blastomeres of the ctenophore egg shows that this need not be the case. [112] In one case a half-embryo resulted. [113] The plane of the first cleavage has been shown in two urodeles to correspond, not to the median longitudinal plane, but to a cross-plane of the embryo. [114] In some cases, especially in sphærechinus, even at the eight-celled stage, the blastomeres seem to shift their position, so that a whole sphere of half size is formed. [115] Hertwig had a year before expressed a similar view in regard to the equivalency of the blastomeres. [116] A view advanced by Pflüger. [117] The evidence to show that more than four and certainly more than eight such groups that come from a single egg can produce a pluteus is, I think, insufficient, and the result improbable.