Anterior 



Lett 



Brain region 



Posterior 



No 13. Early neurula. Dorsol 

 view. Medullary plate stage 



No.14 Neural fold stage. 

 Dorsal view. 



Body 



Head 



Toil 



No 15. Closing neural fold. 

 Dorsal view. 



Gill onlage 



Optic 

 vesici* 



Sucker 



No. 16. Early tail bud. 

 Dorsal view 



Nol7. Eorliest muscular response. 

 Loterol view. 



External gills 



Stage 20. 6mm I40tirs. 

 Gill circulation and 

 hatctiing. 



External gills 



\' 



>>^>//>////^/' 



Ifactory 

 organ 



Stage 23. 9mm 2l6hrs. 



Oifoctory 

 pit, 



Spiracle 



Stage25. Ilmm.284hrs. External gills 

 obsorbed. Left side to show opercular 

 fold and spiracle. 



Fig. 19.8. Survey of the later development of the leopard frog, external views. 

 Stages 13, 14, and 15; nervous system and epidermis. In the neurula stage, the neu- 

 ral or medullary plate extends forward from the blastopore and lies directly above 

 the notochord and adjoining mesoderm. In stage 14 the neural folds are present 

 but are separated by an open trough, the future nerve cord and brain. In 75 the 

 folds gradually close together. The central canal of the mature nerve cord and the 

 ventricles of the brain are remainders of the once open trough. In stages 16 and 

 17, the body is lengthening and the developing muscles twitch spasmodically. The 

 optic vesicles are outgrowths of the brain that form the retinas and optic nerves. 

 The cells of the endodermal lining of the gut are still packed with yolk. Stages 20, 

 23, and 25 show a rapid increase in size due to absorption of water. At hatching, 

 the tadpole is about 56 per cent water; fifteen days after hatching, it reaches its 

 maximum of 96 per cent water. By stage 23 the tadpoles hang by their suckers 

 from submerged stems. By stage 25 they are eating soft plants; their bodies are 

 fish-shaped. The external gills of stage 23 are replaced by internal ones covered 

 by the opercula. The spiracle, a pore on the left side, is the only exit for water. In 

 leopard frogs the respiratory system changes little until the tadpole is transformed 

 into a frog over two months later. (Courtesy, Rugh: The Frog. Philadelphia, The 

 Blakiston Co., 1951.) 



371 



