92 THE FROG. 



* 



At the time of hatching, the cloacal opening is already present ; 

 but the tadpole has no mouth, and is dependent for nutrition, 

 as it has been during all the earlier stages, on the granules of 

 food-yolk contained in the egg itself. A horse-shoe shaped 

 sucker is present on the under surface of the head, by which the 

 tadpole attaches itself, at first to the gelatinous mass of the 

 spawn, and later on to weeds or other objects in the water. 



A few days after hatching, the mouth appears, bordered by 

 a pair of horny jaws, and fringed with fleshy lips studded with 

 horny papillas. The alimentary canal, which has hitherto been 

 short and wide, rapidly increases in length, becoming tubular 

 and convoluted ; the liver and pancreas are formed ; and the 

 tadpole feeds eagerly on confervas and other plants, especially 011 

 decomposing vegetable matter. 



About the time of appearance of the mouth, i.e. shortly 

 after hatching, a series of four slit-like openings, the gill-clefts, 

 appear on each side of the neck, leading from the phaiynx to 

 the exterior. The margins of the slits become folded, and form 

 the internal gills ; the external gills at the same time decreasing 

 in size and becoming shrivelled in appearance. 



While the internal gills are developing, a fold of skin, the 

 operculum, appears on each side of the head, in front of the gills. 

 The two opercular folds, Avhich soon become continuous with 

 each other across the ventral surface of the head, grow back- 

 wards over the gills so as to inclose them in gill-chambers. 

 Towards the end of the fourth week, the hinder edges of the 

 opercular folds fuse with the body wall along the right side and 

 across the ventral surface of the head. On the left side a 

 spout-like opening remains, which communicates with the gill- 

 chambers of both sides ; through this opening the water, taken 

 in at the mouth for respiration, and passed out through the gill- 

 slits, makes its escape to the exterior (cf. Fig. 83). 



During this time the tadpole has been feeding freely, and 

 has greatly increased in size. The body (Fig. 44, 8) is broad 

 and round ; the tail is much larger than before, and forms a 

 powerful swimming organ ; while the sucker on the under 

 surface of the head, though still present, is small, and divided 

 into two separate halves ; and is but little used. 



Very shortly afterwards, rudiments of the hind limbs can be 

 seen as a pair of small papillae at the root of the tail, one on 



