THE DEVELOPMENT OF THE FROG 249 



internal gills, and the external gills begin to atrophy and after 

 a time disappear altogether. Shortly after the formation of 

 the gill-slits a fold of skin grows out from each side of the 

 head and extends backwards, forming a cover or operculum to 

 the gills. The folds unite ventrally and continue to grow 

 backwards so as to form a gill-chamber, and at a later period 

 their hinder edges fuse with the body-wall along the right side 

 and on the ventral surface, but the fusion does not extend to 

 the left side, so an opening is left there, the margins of which 

 are produced into a spout-like process. Through this aperture 

 the water taken in by the mouth and passed through the gill- 

 slits into the gill-chamber is discharged. The tadpole has 

 now attained its maximum development as such. Its body 

 and head are broad and depressed, and it has a large muscular 

 tail provided with a dorsal and a ventral fin-like expansion, 

 used in swimming, but there are no limbs. At this stage of its 

 existence the larval frog or tadpole is a fish, not only in habit 

 but in many important structural features. It has a cartilagin- 

 ous cranium, an unconstricted notochord (in this respect it is in 

 a more primitive condition than the dogfish), gill-slits lined by 

 a vascular membrane serving as the respiratory surface, a 

 bilocular venous heart, afferent and efferent branchial arteries, 

 and the gill-arches are supported by cartilaginous bars, and 

 there is no trace of pentadactyle limbs. But from this point 

 onwards it gradually loses its piscine characters and takes on 

 the structure of an amphibian. Lungs are formed as out- 

 growths of the gullet, and the arterial system is modified in 

 connection with the new mode of respiration ; the gills after 

 a time atrophy and the gill-slits are closed up. The hind 

 limbs are developed on either side of the base of the tail and 

 the fore-limbs beneath the opercular folds, and both pairs 

 gradually assume pentadactyle characters. The tail gradually 

 atrophies and eventually is completely absorbed. The per- 

 manent jaws are formed, the frilled lips shrivel up and the horny 

 jaws are cast off. Finally, in the course of a rapid meta- 

 morphosis, the herbivorous tadpole becomes a carnivorous frog, 

 the intestine becomes shorter, the cartilage of the skeleton is 

 largely replaced by bone, and the animal assumes its adult 

 characteristics. We may now consider some of these remark- 

 able changes in detail. 



The segmentation has already been described (vol. i. p. 122), 



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