INTRODUCTION TO THE EMBRYOLOGY OF THE LEOPARD FROG 21 



also the third germ layer, or the mesoderm, and the notochord, which 

 come from the epiblast. The notochord is the axis around which the 

 vertebral column will be built. The mesoderm will give rise to the 

 bulk of the skeleton and muscle, to the entire circulatory system, and 

 to the epithelium which lines the body cavity. 



Shortly after gastrulation the embryo elongates and develops a dor- 

 sal thickening known as the medullary plate. This thickened ectoderm 



Left 



r^ R R 



Posterior 



No 13. Early neurula. Dorsal No.l4. Neural fold stage. No. 15. Closing neural fold 



view Medullary plate stage Dorsal view. Dorsal view. 



Brain region 



Body 



Toil 



Gill onlage 



No 16. Early tail bud. 

 Dorsal view. 



Sucker 



No, 17. Earliest musculor response. 

 Lateral view. 



External gills 



Stoge 20, 6mm I40hrs 

 Gill circulation and 

 hatching. 



External gills 



Stage 25 llmm 284hrs External gills 

 Spiracie absorbed. Left side to show opercular 



fold and spiracle. 



Early development of the frog embryo. (Top) Development of the axial central 

 nervous system. (Bottom) Development of the external gills and operculum. 



