DEVELOPMENT OF THE FROG 101 



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 respira- 

 tion, and passed through the gill-slits, makes its escape to 

 the exterior, Fig. 20, 5 , c , 8 . It is the inclusion of the 

 gills by this opercular fold that has given rise to the notion 

 of opercular or " internal " gills. True internal gills, how- 

 ever, are not present in the frog or its allies at any stage of 

 development. 



During this time the tadpole has been feeding freely, and 

 has increased greatly in size. The body, Fig. 20, 5 , is broad 

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

 a powerful swimming organ ; while the cement-organ on the 

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

 but little used for adhesion. 



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 

 each side of the anus, Fig. 20, 6 ; these steadily increase in 

 size : about the seventh week they become divided into joints ; 

 and a week later the toes appear, Fig. 20, 7 . 



The fore-limbs arise about the same time as the hind ones, 

 but are covered by the opercular folds, and hence do not 

 become visible till a later stage, Fig. 20, 8 . 



Towards the end of the second month the lungs come into 

 use, and the tadpoles which now have the form shown in 

 Fig. 20, 8 , frequently come to the surface of the water to 

 breathe. The gills now begin to degenerate, but for a 

 time respiration is effected both by the gills and the 

 lungs. 



A fortnight or three weeks later a distinct metamorphosis 

 occurs, whereby the tadpole becomes transformed from the 

 fish-like condition in which it has hitherto remained to the 

 purely air-breathing stage characteristic of the adult. The 

 tadpole ceases to feed ; a casting,' or ecdysis, of the outer 

 layer of the skin takes place ; the gills are gradually absorbed ; 

 the horny jaws are thrown off ; the large frilled lips shrink 

 up ; the mouth loses its rounded suctorial form and becomes 

 much wider ; the tongue, previously small, increases consider- 

 ably in size ; the eyes become larger and more prominent ; 



