STAGING OF AMPHIBIAN EMBRYOS 



65 



NORMAL STAGES OF RANA SYLVATICA 

 STAGES 18 TO 23 (TABLES 2 AND 3) 



The figures are ventral and lateral views of all but stage 22, which shovvs dorsal instead 

 of ventral aspect. 



18. Stage begins with development of capacity for muscular movement, i.e. , simple unilateral flexure in response to mechanical 

 stimulation. This is very suddenly acquired and is closely correlated with attainment of the external form figured. 

 Time given indicates onset of heart beat which appears very suddenly and is accordingly a most useful marker for this stage. 

 (Use of strong reflected light is necessary for identification of this early pulse. ) Tail equals one -third the length of the body. 

 Beginning of circulation of blood corpuscles through a capillary loop of anterior gill is closely correlated with gill morphology, 

 and is the best indication of the beginning of this stage. Shaking will hatch embryos early in this stage; they hatch spontan- 

 eously late in 20. Swimming ability is acquired in the latter part of this stage. Tail equals one-half the body length. 

 Cornea becoming transparent so lens is visible as light spot. Body and tail nearly equal in length. 



Development of posterior bend in gut makes trunk appear asymmetrical from dorsal aspect. A few capillary loops are func- 

 tional in the tail fin. Epidermis rapidly becoming transparent. 



Trunk and head have rounded out and embryo assumes true lan'ol or "tadpole" shope. Horny larval teeth developed. Poster- 

 ior limb bud identifiable. Opercular fold beginning to develop. Active spontaneous swimming begins. 



19. 



20. 



21. 

 22. 



23. 



TABLE 4 



Hours from first cleavage required to 

 reach various stages at different tem- 

 pe rature s 



TABLE 5 



16 



17 



18 



12 



18 



EYE 



EAR 



111 



19 



20 



21 



20 



EYE 



EAR 



(From A. W. Pollister G J. A. Moore) 

 (Anat. Rec. 1937, 68:489) 



INTERNAL ANATOMY (TABLE 5) 



It has been found useful to have some means of readily identifying sectioned material in 

 teriTis of the series of stages described above. There are, of course, no difficulties in 

 doing this with embryos up to the time of closure of the neural folds (stage 15). For 

 recognition of stages 16 to 21 table 5 was constructed. The number of somites was 

 counted from frontal or sagittal sections. The development of the eye and ear, as seen 

 in cross sections, are shown by the series of drawings made at a common magnification 

 by a projection method. 



