STAGING OF AMPHIBIAN EMBRYOS 



89 



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 shows dorsal 

 Instead of ventral aspect. 



18. Stage begins with development of capacity for muscular movement, i.e., simple uni- 

 lateral flexure In response to mechanical stimulation. This Is verj suddenly ac- 

 quired and is closely correlated with attainment of the external form figured. 



19- Time given Indicates onset of heart beat which appears very suddenly and Is accord- 

 ingly a most useful marker for this stage. (Use of strong reflected light is neces- 

 sary for identification of this early pulse.) Tail equals one- third the length of 

 the body. 



20. 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 spontaneously late In 20. Swimming ability is acquired In the latter part of 

 this stage. Tail equals one-half the body length. 



21. Cornea becoming transparent so lens is visible as light spot. Body and tail nearly 

 equal in length. 



22. Development of posterior bend in gut makes trunk appear asymmetrical from dorsal 

 aspect. A few capillary loops are functional In the tail fin. Epidermis rapidly 

 becoming transparent. 



25. Trunk and head have rounded out sind embryo assumes true larval or "tadpole" shape. 

 Horny larval teeth developed. Posterior limb bud identifiable. Opercular fold 

 beginning to develop. Active spontaneous swiimning beglna. 



INTERNAL ANATOMY (TABLE 5) 



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

 in terms of the series of stages described qbove. 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 I6 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 projec- 

 tion method. 



TABLE 4 



Hours from first cleavage required to reach 

 various stages at different temperatures 



TABLE 5 



16 



17 



18 



M 



12 



18 



EYE 



EAR 



19 



20 



21 



ill 



20 



EYE 



^I 



EAR 



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



