BA TEA cm A . 325 



vertebra] axis, goes to sliow tliat, whatever view we may adopt with regard to tlie 

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

 evolved from it or any of its processes. I have aseertain(!d, by <lirect observation, that 

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

 papilla^ 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 jDclvis, and at the same time the papillaj are developed 

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

 appendage to the vertebral coluiini, 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 

 ]3arts in fishes, in which the ventral fins are never connected with the vertebral column 

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



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

 stances, worthy of attention. In the ordinary Ranid;e, the phases of develo])ment 

 ai'e in accordance with the ])eculiar conditions imder which the earlier periods of life are 

 passed ; their habits are not only wholly aquatic, Init 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 Pipa3 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 branchiro are developed, disappear, and are 

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

 full development, with swimming adajitations, 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 branchiaa, 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 Ity an absorp- 

 tion of materials furnished by the dermal sac, since the existence of an oi)erculum 

 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 disai)peared. It seems highly probable that the walls of the 



