NATURAL HISTORY STUDIES 



by means of a paired cement gland below the 

 place where the mouth will appear. A bulging 

 on the under surface of the body shows the position 

 of the still unused remains of the yolk. 



Soon after hatching, three pairs of external gills 

 grow out, the first much the largest, one upon each 

 of the first three gill arches. One or two days after 

 hatching the mouth appears in the centre of a 

 groove in front of the adhesive organ, and hundreds 

 of small horny teeth are formed. When the food- 

 canal becomes open, four pairs of gill-clefts break 

 out from the pharynx, and a gill-cover overlaps the 

 first set of gills. These dwindle and are absorbed, 

 their place being taken by a second set of gills 

 supported by the lower halves of four gill-arches. 



The Fish-likeStage 



About a month after hatching the larval frog is 

 in many ways fish-like : for instance, it has a two- 

 chambered heart which drives impure blood to the 

 gills, which are enclosed by a gill-cover. It swims 

 by its laterally compressed tail, which shows a 

 well-developed unpaired fin, without fin-rays, how- 

 ever, which are always present as supports for the 

 unpaired fins of fishes. In a very general way it 

 may be said that the developing frog visibly climbs 

 up its own genealogical tree. But what took the 

 race of frogs long ages to accomplish is achieved 

 by the individual in a few days a fact so familiar 

 that we are apt to forget its marvellousness. 



With the acquisition of a mouth the larva begins 

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