io8 



NATURE 



[Dec. II, 1873 



met with in the Frog's class, and these only very rarely and 

 in an imperfectly developed condition. 



As has been mentioned, in two kinds of Frog [Cerato- 

 phrys and Ephippifer) the skin of the back is furnished 

 with bony plates. These are found in the deeper layer or 

 dermis, and are therefore '■' scutes." 



The remarkable circumstance, however, is that we have 

 here a lower stage (as it were an incipient condition) of 

 that more developed dermal skeleton which exists in 

 tortoises and turtles. In most of these reptiles both the 

 back and the belly are protected by bony plates which 

 adjoin one another, and together form a solid box in which 

 the body is enclosed. Moreover the bony plates of tor- 

 toises and turtles are invested by large horny epidermal 

 scales (" tortoise-shell "), which scales do not agree in 

 either size or number with the bony plates on which they 

 are superimposed. 



Agam, the middle series of bony plates of the back are 

 continuous with the subjacent joints of the backbones, 

 and the lateral series of dorsal plates are continuous with 

 the ribs beneath them. 



There are certain Chelonians, however — "mud-tortoises" 

 — (of the genus Trionyx), which have the dorsal plates 



F.G. 32. 



Fig. 33. 

 , neural spine ; d^ tubercular 



Fig. 32. — The Axis Vertebra. 



process : p, capitular process : rz, anterior articular surface for 

 z\ postzygapophysis ; fl, odontoid process : hy^ median vertical ridge 

 beneath centrum- FiG. 33. — The Atlas Vertebra, j, rudiment of neural 

 spine : rf, tubercular process : /, capitular process ; a, articular surface 

 for skull : hy^ plate of bone holding the place of a cranium, and articu- 

 lating with the odontoid process of the a.\is vertebra. 



much less developed and not connected with the ventral 

 plates save by means of soft structures. 



Here then we have in reptiles an interesting approxi- 

 mation to the condition we have seen to exist in those 

 exceptional Anourans, Ccratophrys and Ephippifer. More- 

 over this resemblance is still further increased by the fact 

 that in Trionyx the bony plates are not covered with 

 any tortoise-shell, but are merely invested by soft skin as 

 in the genera of dorsally-shielded Batrachians. 



Have we then here a true sign of genetic affinity? Are 

 these tortoises to be deemed the more specially modified 

 descendants of shielded frogs or of some as yet unloiown 

 slightly-shielded animals which were the common ances-" 

 tors both of frogs and tortoises ? 



Certainly tortoises cannot be the direct descendants of 

 frogs, they agree with all reptiles in characters which are 

 both too numerous and too important to allow such an 

 opinion to be entertained for a moment. 



The other opinion is hardly less untenable ; for if all 

 the multitudinous species of fiogs (together with a 

 number of reptilian forms more closely allied to the 

 tortoise than any frogs are) descended from slightly 

 shielded animals, how comes it that all frogs and toads, 

 save one or two species in no other way peculiar, have 

 every one of them lost every trace of such shielded struc- 

 ture which nevertheless cannot easily be conceived to 

 have been in any way prejudicial to their existence and 

 survival ? 



On the other hand, it cannot but strike us with surprise 

 that structures so similar — extending even to the con- 

 tinuity of the dorsal plates with the subjacent joints of 

 the backbone — should have arisen twice in nature spon- 

 taneously. Here \vc seem to have a remarkable example 

 of the independent origin of closely similar structures ; 

 and if so, what caution is not necessary before concluding 



that any given similarity of structure are undoubted 

 marks of genetic affinity ! 



The skin of the frog is also interesting from a physio- 

 logical point of view. Our own skin is by no means 

 popularly credited with the great importance really due 

 to it. "Only the skin!" is an exclamation not unfre- 

 quently heard, and wonder is very often felt when death 

 supervenes after a burn which has injured but a com- 

 paratively small surface of the body. Yet our skin is 



Fig. 34. — Lateral, Dorsal, and Ventral view of first Vertebra Qi Aiptpk. 



really one of our most important organs, and is able to 

 supplement, and to a very slight extent even to replace, 

 the respective actions of the kidneys, the liver, and the 

 lungs. * 



In the frog we have this cutaneous activity developed 

 in a much higher degree. Not only does \\.i perspiratory 

 action take place to such an extreme degree that a frog 

 tied where it cannot escape the rays of a summer's sun 

 speedily dies — nay, more, is soon perfectly dried up — 

 but its respiratory action is both constant and important. 

 This has been experimentally demonstrated by the de- 

 tection of the carbonic acid given out in water by a frog 

 over the head of which a bladder had been so tightly tied 

 as to prevent the possibility of the escape of any exhal- 

 ation from the lungs. The fact of cutaneous respiration 

 has also been proved by the experiment of confining frogs 

 in cages under water for more than two months and a 

 half, and by the cutting out of the lungs, the creature 

 continuing to live without them for forty days. Indeed 

 it is now certain that the skin is so important an agent in 

 the frog's breathing that the lungs do not suffice for the 

 maintenance of life without its aid. 



It is no less true that in Batrachians which breathe by 

 means of permanent gills — as, e.g. the Axolotl — such 

 gills are not necessary to life, as the late M. Aug. Dumeril 

 and Dr. Gunther have established by cutting them away 

 without inducing any apparent injurious effects. In the 

 whole class of Batrachians skin respiration seems, then, 

 to be of very great importance. 



F'c- 35- Fig. 36. 



Fig. 35.— Coccy.\ of Frog, lateral \ieu-, a ijlack line indicates the course of 

 the sciatic nerve. Flc. 36.— Anterior aspect of Coccyx, showirg the 

 double articular concavities placed side by side beneath the neural arch. 



The internal skeleton (or the skeleton commonly so 

 called) of the frog presents soine points of considerable 

 interest, especially as exhibiting its Intermediate position 

 between fishes on the one hand, and hiijher vertebrates 

 on the other. First, as regards the backbone, it may be 

 remembered that it is made up of distinct bony joints (or 

 vertebnc), in which it agrees with all animals above fishes 

 and with bony fishes ; its hinder termination, however, is 

 essentially fish-like. It is fish-like, because the terminal 

 piece, as it is called, or ''eoeeyi" (unlike the coccyx in 

 man or in birds) is not formed of rudimentary vertebra; 

 which subsequently blend and anchylose together, but is 

 formed by the ossification continuously of the membrane 

 investing (or sheath of) the hindermost part of that primi- 

 tive continuous rod, or notochord, which, as has been said, 



* See " Elementary Physlolo^', ' Lesson V., § 19. 

 t From NiIiToc, back, and \op6n, chord. 



