VII.] 



THE COMMON FROG. 



99 



with a mass of muscular fibres not arranged in super- 

 imposed sheets, but as a series of narrow segments 

 'separated from each other by layers of membrane. 

 The edges of these membranous layers, when the skin 

 is removed, appear as a successive series of undulating 

 lines proceeding from the back to the belly. 



Fig. 62.— Tadpole of Bull Frog, partly dissected, to show the muscles of the tail and 

 the branches of the 8th ner^'e or tne vagtts. a, great lateral branch giving off — 

 b, a dorsal branch, and c, the lateral branch (or ne>-viis lateralis) ; d, branches 

 descending and passing r«'ong the branchial arches. The de'^cending branches 

 seen behind the branchial nerves on the side of the belly are not branches of the 

 vagus at all, but spinal nerves, which come out from beneath the muscles and 

 pass down under the nervus lateralis, and without having any communication 

 with it. 



Now the tadpole exhibits a muscular condition 

 quite similar to that of the fish, and in the great 

 persistent larva the axolotl, we find no truly oblique 

 abdominal muscles, but only as it were a hyper- 

 trophied rectus. 



In other species of the frog's class, which retains a 

 tail throughout life, the marked superimposed lamellae 

 are distinctly developed, but more or less' distinct 

 traces are also retained of the successive membranous 

 partitions separating the muscular segments of both 

 the dorsal and ventral regions. 



Another stage of development may be detected in 

 the tail-muscles of certain reptiles. 



Here the membranous partitions have become 

 drawn out at short intervals from above downwards 

 into a funnel-shaped condition, so that the muscular 



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