BATRACHIA. 325 



vertebral axis, goes to show that, whatever view we may adopt with regard to the 

 homology of legs, the pelvis included, they are something superadded to it, and not 

 evolved from it or any of its processes. I have ascertained, by direct observation, that 

 even among frogs, the legs, which appear on each side of the tail in the form of small 

 papillae, are primarily tegumentary growths, that beneath these there is developed a 

 cartilaginous plate which gradually extends upwards on each side, until it meets with 

 the transverse process of the vertebral column, with which it becomes permanently con- 

 nected under the form of the pelvis, and at the same time the papillae are developed 

 into limbs with their continued bones ; thus the pelvis, which in the adult seems to be an 

 appendage to the vertebral column, is, in the embryo, an independent structure, just 

 as the tooth is primarily independent of the jaw. In this mode of the development 

 of the legs, we have a temporary analogy to the permanent constitution of the same 

 parts in fishes, in which the ventral fins are never connected with the vertebral column 

 by their pelvic bones, these being confined to the abdominal surface of the body. 



" The complete development of the tail adapted to swimming is, under the circum- 

 stances, worthy of attention. In the ordinary Ranidae, the phases of development 

 are in accordance with the peculiar conditions under which the earlier periods of life are 

 passed ; their habits are not only wholly aquatic, but they have many of the anatomical 

 and physiological characters of fishes, among which may be mentioned the existence 

 of branchiae for aquatic respiration, and a broad and compressed tail for aquatic loco- 

 motion. The embryos of Pipae differ from those, of other allied genera, in passing 

 through all of their embryonic phases in closed dermal sacks, where they neither 

 breathe by the action of aquatic currents, nor are capable of executing the ordinary 

 locomotive movements ; yet, the external branchiae are developed, disappear, and are 

 replaced by internal branchiae, and these in turn by lungs ; the tail also requires its 

 full development, with swimming adaptations, in the form of muscles and folds of 

 skin, as in other tadpoles, and, after having existed for a certain period, is removed by 

 absorption, without having been once made use of as a locomotive organ. It appears 

 that, in this particular instance, the exigencies of embryonic life do not require the 

 existence of a tail for the purposes of locomotion, and its presence seems to be ac- 

 counted for only on the supposition of the existence of a preestablished plan, accord- 

 ing to which Batrachians generally are developed; and this plan is adhered to, although 

 the organ may not be used, or not used in the same way as in the other species. 



" It is possible that the materials of the tail serve as a store of nutritive substance, 

 though this seems scarcely probable ; but, even if this be the case, it is none the less 

 a fact that the part assumes a structure, the adaptations of which have reference to a 

 function wholly different. As regards the existence of branchiae, I have observed an 

 analogous instance in the embryos of Plethodon cinereus, where these organs are 

 developed externally, though the eggs are deposited under a log, and the animal is 

 not aquatic at any period of its life. 



" The only other subject to which it is proposed to refer is the growth of the em- 

 bryo, by which there is formed, at the end of incubation, a larger mass than existed 

 in the egg when it commenced. 



" This increase in bulk could have been effected in no other way than by an absorp- 

 tion of materials furnished by the dermal sac, since the existence of an operculum 

 . would prevent the entrance of nutritive matter from without. The gelatinous matter 

 which originally surrounded the egg may have contributed something, but still there 

 is growth after this has disappeared. It seems highly probable that the walls of the 



