INTRODUCTION TO THE EMBRYOLOGY OF THE LEOPARD FROG 23 



achieved, the organ systems are not all developed to the adult form. 

 The external gills of the tadpole function for a short time and then 

 are replaced by internal gills. The external gills are covered by an 

 operculum or posterior growth of the hyoid arch, with but a small pore 

 or spiracle remaining on the left side of the head. This is the only 

 channel of egress for water from the pharynx and out over the internal 

 gills within the branchial chamber. 



From the first day of hatching, when the total water content of the 

 tadpole is about 56 per cent, there is a very rapid rise in the water con- 

 tent until the fifteenth day after hatching, when it reaches the maxi- 



-Toil bud 



Stage 18 

 4mm. 



Stomodeal cleft 



-Gill bud 



Olfactory pit 



Stage 19 

 5mm. 



Mouth 



External 



gills 



Opercular 

 fold 



Stage 23 

 9 mm. 



Mouth 



Oral 

 sucker - 



Stage 20 

 6mm. 



Stage 21 

 7mm. 



-Mouth 



Stage 22 

 8 mm. 



Remnant, 

 oral sucker 



Spiracle 



Development and absorption of the external gills of the frog larva. 



