64 



STAGING OF AMPHIBIAN EMBRYOS 



NORMAL STAGES OF RANA SYLVATICA 



DESCRIPTION 

 STAGES 1 TO 



OF STAGES* 

 17 (TABLE 1) 



TABLE 1 





5T?«: EXTERNAL 



1^ 



1. Egg at fertilization. 



2. Establishment of gray crescent area as first external evidence of 

 development, sharply defined at 1 hour. 



3 to 6. Age given is time of appearance of cleavage furrow that 

 establishes the number of cells drawn for the stage. 



7 to 9. Later stages in cell multiplication, best determined by com- 

 parison of size of cells at vegetal pole. 



10. Appearance of dorsal lip. 



11. Blastopore approximately a semicircle. 



12. Complete blastopore (yolk-plug) stage. 



13. Slit blastopore or neural plate stage. 



14. Neurol fold stage. 



15. Beginning of closure of neural folds, beginning of elongation. 

 Cilia begin to rotate the embryo at about this stage. 



16. Closure of neural folds completed. 



17. Beginning of development of tail bud, marked off from body by 

 ventral notch when embryo is viewed laterally. 



* The stages illustrated and defined by age are of course essentially 

 an arbitrary series of readily identifiable points in the continuous 

 process of development. Defined in terms of these points, the 

 total development is comprised in a series of periods, each ex- 

 tending from one stage until the next. In practice the points and 

 the periods are not sharply distinguished and it is convenient to 

 describe each period in terms of the stage that initiates it. Thus 

 the development from onset of the heart beat to the beginning of 

 gill circulation would be the period of stage 19. 



TABLE 2 



TABLE 3 



EXTERNAL TORM 



MUSCULAR MOVEMENT 



HCAAT SCAT 



GIUL CinOJiATtON 

 SWIMMfNC- HATCHtHC 



as 



IS 



EXTERNAL FORM 



CORNtA TRANSPARENT 



TADratC rORM 

 TICTH 



LIMB euo 



(From A. W. PoUister C J. A. Moore) 



