394 THE MECHANICAL FUNCTIONS. 



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 rotatory 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 accomplished. By carefully watch- 

 ing 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 ver- 

 tebra 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, which being joined 

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

 deformity 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 portion of the spinal marrow which had passed 

 through it in the aquatic life of the animal being 

 now withdrawn. 



The theory of the spinal origin of the cranial 



