552 



AMPHIBIA. 



external gills soon become branched. There are three 

 of them on each side, the first 

 the largest. The mouth, which 

 has previously been merely a blind 

 pit, opens into the gut, the gut 

 itself lengthens rapidly, and be- 

 -na comes coiled like a watch-spring ; 

 1"° the larvae feed eagerly on veget- 

 ^J D able matter and increase in size. 

 The glandular crescent forms two 

 small discs, which gradually dis- 

 appear as the power of locomotion 

 increases. About the time when 

 the mouth is opened, four gill- 

 clefts open from the pharynx to 

 the exterior. 



A second period, the true tad- 

 pole stage, now begins. A skin-fold 

 or operculum covers the external 

 gills, which then atrophy, and are 

 replaced by internal gills developed 

 on four branchial arches. The 

 mouth acquires horny jaws, and 

 the fleshy lips bear horny papillae. 

 By the continued growth of the 

 opercular folds the gill-chambers 

 are closed, with the exception of 

 a single exhalant aperture on the 

 left side. Through this opening, 

 the water which is taken in by the 



„ , mouth in respiration passes out- 

 Fig. 238. — Dissection of tad- j i_ • , ,;. .„ 



pole?- After Milnes Mar- wards, having washed the gills on 



shall and Bles. its way. 



dl., lower lip; #., ventricle of i n tne third period the rudi- 



heart ; Z>E. t oesophagus; NA., , f 



head kidney; a., aorta; k., ments or the limbs appear. The 

 ™»ZU%%T!&-, kv.; fore-limbs are concealed within the 

 opening of ureter into cloaca ; gill-chambers, and so are not 



GR., genital ridge; G.F., fatty 1 • .., , , . 



body; lf., forf-iimb; oa, in- obvious until a later stage; but 

 SL g '" s; "' epidermis; *' the hind-legs may be watched 

 in the progress of development 

 from small papillae to the complete limb. 



