ONTOGENY 



255 



Fig. 157. — Larval stage of Rana 

 arvalis, ventral view, in which the 

 suckers (d.s.) are degenerating and the 

 mouth is provided with horny jaws. 

 {After Lieberkind.) 



another between the second and third. The clefts are narrow 



slits connecting the pharynx with the outside of the body. The 



gill slits develop from gill 



pouches. These are lateral out- 



pocketings of the pharynx, sepa- 

 rated from each other by gill 



arches. A total of six pouches 



are formed, of which the first and 



last remain rudimentary. Each 



of the remaining four on each 



side enlarges until the endoder- 



mal lining comes in contact with 



the ectoderm when the tissue 



breaks down to form a vertical 



slit. The external gills now become enclosed in a branchial 



chamber by the operculum, which is formed of two folds of 



skin, one on each side, from the region in front and above the 



first pair of gills. These two folds grow 

 backward over the gills, meet, and fuse 

 with each other ventrally and with the 

 body wall posteriorly, forming a common 

 chamber communicating with the outside 

 by a small opening, the spiracle, on the left 

 -6 side. The external gills are now resorbed 

 and replaced by internal gills developed 

 from the borders of the gill clefts. The 

 internal gills are aerated by water taken 

 in the mouth and passed through the gill 

 clefts into the branchial chamber and out 

 through the spiracle. The gills and skin 

 Fig. 158. Larval stage are ^^g principal organs of respiration 



of Rana arvalis, ventral . 



view, showing gills (g) during the larval period (approximately 

 still visible beyond o P er- t h ree months). The lungs develop as 



culum on left side. On 



the right side the gills are outgrowths from the floor of the pharynx 

 enclosed in the bran- anc j are r eady to take over the respiratory 



chial chamber formed by 



the operculum. (After function by the end of the larval period 



Lieberkind.) (Fiff 158) 



Paired Appendages. — During the larval period the caudal 

 region of the tadpole increases in size and is provided with a well- 

 developed vertical fin. The development of the legs begins at 



