DEVELOPMENT OF THE FROG 111 



gills, followed shortly by a third pair, grow out from the sides 

 of the neck, and in about a fortnight from the time of laying of 

 the eggs the young tadpoles make their way out of the gelatinous 

 mass of the spawn, and swim freely in the water. 



At the time of hatching the tadpole has no mouth, and is 

 dependent for food on granules of food yolk which are contained 

 in large numbers in the egg, and at the expense of which all the 

 earlier processes of development are effected. A horse-shoe 

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

 which the tadpole is enabled to attach itself 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 provided with 

 horny papillae. The alimentary canal which has hitherto been 

 wide and short, now rapidly increases in length, becoming 

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

 anus is developed even before the mouth, and the tadpole now 

 feeds eagerly on confervse and other 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 pharynx to the 

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

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

 in size and assuming a shrivelled 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, 

 and grows backwards over these, so as to enclose them in a gill 

 chamber. Towards the end of the fourth week the hinder edges 

 of the opercular folds fuse with the body wall on the ventral 

 surface and along the right side. On the left side a spout-like 

 opening remains which communicates with the gill chambers of 

 both sides, and through which the water taken in at the mouth 

 for respiration, and passed through the gill slits, makes its 

 escape to the exterior. 



During this time the tadpole has been feeding freely, and 

 has increased greatly in size. The body, Fig. 21, 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 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 



