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Development of a frog. 1, two cell stage ; 2, four cell stage ; 3, 8 cells are formed, 

 notice the upper cells are smaller ; in (4) the lower cells are seen to be much 

 , larger because of the yolk ; 5, the egg has continued to divide and has formed 

 a gastrula; C, 7, the body is lengthening, head is seen at the right hand end; 

 8, the young tadpole with external gills; 9, 10, the gills are internal, liitid legs 

 beginning to form; 11, the hind legs show plainly; 12, i:j. 14, la^or stages in 

 development; 15, the adult frog. Figures 1, 2, .S. 4. .'>, (\, and 7 are very 

 much enlarged. (Drawn after Leukart and Kny by Frank M. Wheat.) 



245 



