166 INTERNAL DEVELOPMENT. 



into a vertebral column, connected throughout by the true ball- 

 and-socket joint characteristic of the higher reptiles. 



The changes which take place in the circulatory and breathing 

 apparatus are not a whit less remarkable. In the first instance 

 the condition of these organs is in all essential particulars 

 precisely the same as in the class of fishes. Thus the blood, 

 driven from the arterial bulb through the branchial art/^ies, 

 circulates freely through the gills, and is again collected in the 

 branchial veins, of which two pairs unite in forming the great 

 aorta of the body, whilst the other pair is distributed in the head. 

 The pulmonary arteries first make their appearance in a very 

 rudimentary form, springing from the branchial vessels. But as 

 the lungs are developed and aerial respiration commences, these 

 arteries rapidly increase in size, whilst the branchia and their 

 vessels gradually diminish in the same proportion. The change 

 still proceeding, the branches which unite the branchial arteries 

 acquire a much greater development, and gradually divert more 

 and more of the blood from the branchiae, which now speedily 

 disappear altogether, and leave to the pulmonary arteries the task 

 of carrying on the entire business of respiration. In this way the 

 little fish-like Tadpole, adapted exclusively for an aquatic life, 

 gradually becomes metamorphozed into a being possessing every 

 requisite for a strictly terrestrial existence ; though, whether it be 

 from the strength of early associations, or from some more re- 

 condite cause, the Frog seldom wanders far from his original 

 home, and appears his life through to prefer to either land or water, 

 strictly so called, that convenient compromise between the two 

 afforded by the swamp or ditch. 



The progressive changes related above obtain of course only 

 in the young of the higher grades of Batrachians. In the lower 

 forms the metamorphosis is arrested at various points, so that 

 the mature condition of these animals marks, so to speak, the 

 successive stages in the development of the young of the more 

 advanced members of the order. 



But there are some rather curious and some truly extraordinary 

 circumstances in connection with the early history of some of 

 these higher members which must be referred to here. In the 

 rst place, there is a Toad common enough throughout the 

 Continent, and well known in the neighbourhood of Paris, which 

 has obtained for itself the name of the Accoucheur Toad (Bufo 



