60 Royal Institution. 



takes place in the progress of an organism from its commencement 

 to maturity. This was exemphfied by reference to the human sub- 

 ject, to the Hon which acquires its mane, to the stag which gets its 

 horns, and to the change of plumage in birds during the course of 

 growth. The changes of form and character are still more remarkable 

 in the kangaroo ; and in the frog they are such as to have received 

 the name of ' metamorphosis.^ 



The development of the frog was traced to its exclusion from the 

 egg in the form of a fish, with external gills, a long caudal fin, and 

 without legs. 



The internal skeleton, like the external shape, is adapted for 

 aquatic life. 



Only those parts are ossified which are to be retained in the 

 mature state. The vertebrae are at first biconcave, as in fishes, with 

 invervening spherical elastic balls filled with fluid : they are converted 

 into ball and socket joints by the ossification of the sphere, and its 

 anchylosis to the back part of the vertebrae. The pelvis and hind 

 legs are progressively developed ; and, whilst this change is pro- 

 ceeding, the tail is undergoing proportional absorption. The chief 

 change in the skull of the larva is operated in the lower or haemal 

 arches and their appendages. The maxillary arch is widened and 

 provided with teeth, and the horny mandibles are shed. The man- 

 dibular arch retrogrades as well as expands. The hyoidean undergoes 

 a remarkable change of size and shape, and the branchial arches 

 are absorbed, excepting a small portion which is converted into the 

 hinder ' horns ' of the hyoid for supporting the larynx. 



The scapular arch, which at first was connected with the occiput, 

 whilst supporting the branchial heart — its primary function, begins, 

 as soon as the fore legs bud out, to retrograde, and the sternum is 

 developed to complete the ' point d'appui ' for the fore limbs. 



The food of the larva is chiefly the soft decaying parts of aquatic 

 plants ; it has a horny beak, a long alimentary canal disposed in a 

 series of double spiral coils : but, as its frame undergoes the changes 

 adapting it for life on land, and a purely animal diet, the mandibles 

 are converted into jaws and teeth, and the long spiral intestine into a 

 short and slightly convoluted one. 



Soon after the external gills have reached their full development 

 they begin to shrink and finally disappear ; but the branchial circu- 

 lation is maintained some time longer upon internal gills : by ana- 

 stomoses between the principal branchial vessels these are converted 

 into the aortic arches, carotids and subclavians ; the internal gills 

 with the cartilaginous hoops supporting them are absorbed, and 

 lungs and glottis for breathing the air directly are developed. 



Thus an animal formed for moving in water is changed into one 

 adapted for moving and leaping on land ; a water-breather is con- 

 verted into an air-breather ; a vegetable feeder into a carnivorous 

 animal : yet the series of transmutations are limited to the nature 

 of the species and produce no other. The frogs that croak in our 

 marshes are as strictly batrachian as those that leapt in Pharaoh's 

 chamber ; their metamorphoses haAc led to nothing higher than their 



