SKELETON OF THE BATRACHIA. 307 



differs widely from what we have seen in fishes, and ap- 

 proaches to the structure of tlie higher chisses of vertel)rata. 

 The body of each vertebra, instead of having at its posterior 

 surface a cup-like cavity, terminates by a projecting ball, 

 which is received into the cavity in the anterior surface of 

 the next vertebra, so as to compose a true ball and socket 

 joint, capable, when other circumstances permit, of a rotato- 

 ry motion. But the vertebrae of the tadpole, as we have 

 seen, are constructed on the model of those of a fish; that is, 

 have cup-like cavities on both their surfaces, which play on 

 balls of soft elastic matter, interposed between them. We 

 should naturally be curious to learn the mode in which the 

 transition from this structure to that of the frog is accom- 

 plished. By carefully watching the progress of ossification, 

 while this change is taking place, Dutrochet found that the 

 gelatinous ball, on which both the adjacent vertebrae play in 

 the tadpole, becomes gradually more solid, and is converted 

 into cartilage. This cartilage afterwards becomes united by 

 its anterior surface to the vertebra which is in front of it; 

 and the whole then becomes ossified, so as to compose only 

 one bone, its posterior surface remaining distinct, and con- 

 tinuing to play within the cup-like hollow of the vertebra 

 which is behind it. The cartilaginous coccygeal vertebrae 

 of the tadpole are lost long before there is time for their 

 being ossified; but those nearest to the body are consolidated 

 into one long and straight os coccygis, whicii, being joined 

 to the sacrum at an angle, gives rise to the strange deformi- 

 ty observable at that part of the back of a frog; for it here 

 looks as if it had been broken. The spinal cavity is, at the 

 same time, obliterated; that j)ortion of the spinal marrow 

 which had passed through it, in the aquatic life of the ani- 

 mal, being now withdrawn. * 



The theory of the spinal origin of the cranial bones re- 

 ceives considerable support from their structure and relative 

 position in the skeleton of the frog. The cavity for the 

 lodgement of the brain, which is enclosed by these vertebrae, 

 is perfectly continuous, in the same line with the spinal ca- 



