GENERAL ACCOUNT OF DEVELOPMENT 



93 



each side of the cloacal opening (Fig- 71); the limbs increase 

 steadily in size ; about the seventh week they become divided 

 into joints, and a week or so later the toes appear. The fore 

 limbs arise about the same time as the hind limbs, but are 

 covered by the opercular folds, and hence do not become visible 

 until a later stage (Figs. 84 and 85, la). 



Towards the end of the second month the lungs come into 

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

 44, 9 and 10, frequently come to the surface of the water to 



Fig. 44. 



-Various stages in the development of the Froj 

 (From Brehm's ' Thierleben.') 



1, eggs just laid. 2, eggs shortly after laying. 3, tadpole shortly before hatching. 

 4. tadpoles just hatched. 5 and 6, tadpoles with external gills. 7 and 8, tadpoles with 

 fully formed opercular folds. 9 and 10, tadpoles with wed-developed hind legs, shortly 

 before the metamorphosis. 11, tadpole during the metamorphosis. 12, young frog 

 with tail only partially absorbed. 



breathe. The gills begin to degenerate, but for some time 

 respiration is carried on 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 been, to the purely air- 

 breathing state characteristic of the adult. The tadpole ceases 

 to feed ; a casting, or ecdysis, of the outer layer of the skin 

 takes place; 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 



