92 THE FROG. 



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

 but the tadpole has no mouth, a nd is dependent for nutritio n, 

 as it has bee n during all th e earlier stages, on the granules of 

 food-yolk c ontained in the egg itseif. A horse-sboe shap ed 

 s ucker i s present on the under surface o f the head, by which the 

 tadpole attaches itself, at first to the gelatinous m ass of the 

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



A few days after hatching, the mouth app ears, bordered by 

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

 homy papUlas. The alimentar y- cana l, which has hitherto been 

 short and wide, rapidly increases in length, becoming tubular 

 and convoluted; the liver and pa ncrea s are formed; and the 

 tadpole feeds eagerly on confervse and other plants, especially on 

 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-cle fts, 

 appear on each side of the neck, leading from the phar ynx to 

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

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

 in size and becoming shrivelled in appearance. 



While the i nternal g-ill s a*e developing, a fold of skin, the 

 o percul um, appears on each side of the head, in front of the gills. 

 The two opercular folds, which 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 g Ul-chambe rs. 

 Towards the end of the fourth week, the hinder e3gS~'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-bke 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, s) 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, rudime nts of the hind li mbs can be 

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



