232 



GASTRULA-ARRESTED EMBRYOS 



Stage Number 



Age -Hours at Ib'C 







UNFERTILIZED 



Stage Number 



Age -Hours at I6X 



7.5 



32-CELL 



Stage Number 



Age- Hours AT ISX 



13 



50 



NEURAL PLATP 



6 



14- 



62 



GRAY CRESCENT 



MID-CLEAVAGE 



NEURAL FOLDS 



3.5 



4.5 



5.1 



TWO- CELL 



FOUR- CELL 



9 



21 



10 



II 



6.5 



EIGHT- CELL 



SIXTEEN-CELL 



LATE CLEAVAGE 



26 



34 



DORSAL LIP 



MID-OASTRULA 



42 



LATE GASTRULA 



61 



ROTATION 



12 



64 



NEURAL TUBE 



TAIL BUD 



Fig. 1. Normal stages in the development of embryos in the species 

 Rana pipiens. (From Shumway, 1940.) 



