100 DEVELOPMENT OF THE FROG 



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. 



The egg is surrounded by a tough membrane through 

 which the tadpole has to find a way. The method of hatch- 

 ing appears to be as follows. The tadpole possesses a frontal 

 gland immediately in front of the horseshoe-shaped cement- 

 organ on the under side of its head. Fixing this cement- 

 organ to one point of the egg-membrane, it performs 

 movements around this fixed point by the aid of the cilia 

 which cover the greater part of its body, and rubs the 

 frontal gland over this part of the vitelline membrane. 

 Ultimately the membrane is softened and gives way, allowing 

 the tadpole to creep out and adhere to weeds. 



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 ex- 

 pense of which all the earlier processes of development are 

 effected. 



A few days after hatching the mouth appears, bordered 

 by a pair of horny jaws, and fringed with fleshy lips pro- 

 vided with horny papillae. The alimentary canal which has 

 hitherto been wide and short, now rapidly increases in length, 

 becoming tubular and convoluted in the form of a reversed 

 spiral ; the liver and pancreas are formed ; the anus is de- 

 veloped even before the mouth, and the tadpole now feeds 

 eagerly on confervas 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 ectoderm covering them becomes tasselled 

 and forms a series of opercular gills. At the same time the 

 earlier formed more dorsally placed external gills degenerate. 



While external gills bordering the gill-slits 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 



