THE DEVELOPMENT OF AMPHIBIA. 



165 



In the Satrachia, as development proceeds, tlie external 

 branchiae disappear, and are succeeded, functionally, by short 

 branchial filaments developed upon the whole length of each 

 of the branchial arches, of which there are four. 



Fio. 60. — A. B., Tadpoles with external branchiae: 7i, nasal sacs; a, eye; o, ear; k 5, bran" 



chi£e ; to, mouth ; «, horny jaws ; s, suckers ; <?, opercular fold. 

 C, a more advanced Frog's larva : y, the rudiment of the hind-limb ; k s, the single branchial 



aperture. The figure has not been reversed, so that this aperture appears to he on the 



right side instead, of the left. 



Before the development of the lungs the heart has only 

 a single auricle ; afterward, the auricle becomes divided into 

 two. The aortic arches, at first, pass along the visceral and 

 branchial arches to the dorsal aorta, as in other vertebrate 

 embryos. When external gills are developed, each receives a 

 loop from the corresponding arch, much as in Proteus, 



When the internal gills of the Patrachia appear, each 

 aortic arch which belongs to a branchial arch splits into two 

 trunks, — one which remains directly connected with the cardiac 

 aorta, and another which opens into the dorsal aorta. The 

 vessels of the branchial filaments constitute loops between 

 these afferent and efferent trunks, which always remain united 

 by anastomoses. When branchial respiration ceases, and the 

 branchial processes and their vessels disappear, the anasto- 

 moses dilate ; the direct communication between the afferent 

 and efferent trunks of the second pair of internal branchiae is 

 reestablished ; and they become the permanent arches of the 



