Chap. II.] 



EXTERNAL CHARACTERS. 



angles of which are the nasal pits (na.). The position of 

 eye (opt.) and ear (au.) are marked out. At the sides of the head 

 are four bar-like elevations, indicating the position of so-called 

 visceral arches. The first (md.) is called the mandibular arch ; 



Fro. 2. METAMORPHOSIS OF FROG. 



i.-v. Stages in development of tadpole, au. Auditory organ, bl. Blastopore. 

 Ir. i.-ii. First and second branchial arches. 5r. a. Branchial aperture. 

 ex. Ir. External branchiee. /. I. Fore-limb, fr. Frontal process, h. I. Hind- 

 limb, liy. Hyoidean arch. m. Mouth, n. f. Neural fold. n. g. Neural groove. 

 md. Mandibular arch. na. Nasal pit. op. Operculum. opt. Optic pit. s. 

 Suckers, v. Vent. 



the second (%.) the hyoidean ; the other two are branchial 

 arches, and bear the rudiments of external gills, or branchia3. 

 Posteriorly, beneath the root of the tail, is a depression for the 

 vent (proctodceum, v.). 



