Dec. II, 1873] 



NA TURE 



107 



THE 



COMMON 

 VI. 



FROG'' 



The Skeleton of the Frog 

 T T may cause surprise to speak of the skin of the com- 

 ■'■ mon Frog as part of its skeleton, consisting as the 

 skin does of soft membranous structures only. 



The term "skeleton," however, should properly include 

 all the membranous and gristly, as well as the bony struc- 



FiG. 28.— Diagram of a vertical section of both Carapace and Plastron of a 

 Tortoise, made transversely to the long axis of the skeleton c. vertebral 

 centrum : ns, neural spine which expands above into a median dorsal 

 scute ; r, rib which forms one mass with a lateral scute and terminates 

 at a marginal plate ; ic, inter-clavicular scute ; hy, hyo-stemal scute. 



under the name " skeleton," even in the popular signifi- 

 cation of that term. 



Fig. 27. — Dorsal surface of the Carapace of a Fresh-water Tortoise (Emvs) 

 1--8, expandf-d neutral spines; r' — r^, expandptl ribs : nit, first median 

 (or nuchal) plate ; fy, last median (or pygall plate : >«. marginal scutes 

 The dark lines indicate the limits of the plates of the horny epidermal 

 tortoise-shell ; the thin sutures indicate the lines at the junction of the 



tures.* Moreover, more or less of the skin may attain to 

 so solid a condition as fully to justify its comprehension 



The skin of Vertebrate animals consists of two layers : 

 an outer layer (the epidermis or ecteron), and an inner 

 layer (the dennis or enderon). The epidermis, and any 

 projections or piocesses developed from it when they 

 take on a dense or hardened structure, become horny. Of 

 such horny nature are hairs, feathers, nails, and scales, 

 they are more or less dense epidermal appendages. The 



( Triojtyx), showing ties dorsalj laces. 



dermis when hardened becomes bony, and of such nature 

 are the bony skin- plates or " scutes," and teeth. They are 

 dermal appendages. Now both layers of the skin of tha 

 common Frog are entirely soft and utterly destitute of any 



* Continued fron 

 t See " Lessons i 



[ Elementary Anatomy," Lesson IL, 



Fig. 30.— Backbo 



of the Frog (dorsal aspect), 

 the Frog (ventral aspect). 



Fig. 31. 



Fig. 31. -Backbone of 



of these appendages. Allied forms, however, present us 

 with examples of some interesting epidermal conditions , 

 Thus in old male Toads, in Dactylethra and in one of the 

 Japanese efts, the epidermis of some of the finger-tips 

 becomes hardened and horny, in other words we begin to' 

 meet with incipient " nails." " Incipient " because, in 

 ascending from the lowest vertebrates, " nails " are first 



