1 66 COMPARISONS OF STRUCTURE IN ANIMALS. 



posed between the bodies of each, allows of 

 considerable facility of motion. The tadpole, 

 as we know, is furnished with a tail or caudal 

 paddle, by the action of which it propels 

 itself in the water ; through this tail the 

 vertebra are also continued. But a singular 

 and very surprising change soon takes place, 

 not only in the organs of respiration, (which 

 are at first gills,) but in the totahty of the 

 osseous framework. First, the liind-limbs 

 begin to bud forth, and when these have 

 assumed something like a definite shape, 

 though still very small, the anterior limbs 

 begin to germinate, and gradually assume 

 their due form and proportions. While this 

 is going on, a change is taking place in the 

 spine : the tail is gradually absorbed, the 

 process of absorption commencing at the tip, 

 and proceeding gradually to the base, tUl 

 the whole disappears. The spine is at the 

 same time losing its fish-like mode of articu- 

 lation. The ball of elastic gelatine, which 

 interposed between each of the vertebr£e, fill- 

 ing the cup-like cavities of the two in juxta- 

 position, and giving them liberty of motion, 

 or rather a sort of spring-hke action, becomes 

 converted into curtilage ; and this cartilage 



