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 yoke ; 5, the egg has continued to divide and has formed 

 a gastrula ; 6, 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, hind legs 

 beginning to form; 11, the hind legs show plainly; 12, 13, 14, later stages in 

 development; 15, the adult frog. Figures 1, 2, 3, 4, 5, 6, and 7 are very 

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



24o 



