88 



STAGING OF AMPHIBIAN EMBRYOS 



NORMAL STAGES OF RANA SYLVATICA 

 TABLE I TABLE 2 



STj^ EXTERNAL 

 -a? rORM 



51 Si EXTERNAL 

 -JOfVl FORM 



st|^' external 



FORM 



VesJ>/ 



o 



NiVisi-^ 



Vj^^ 



EXTERNAL FORM 



MUSCULAR MOVCMeNT 



CIIX CincULATION 

 SWIUUrNC- HATCHING 



TABLE 3 



r^^f 



V. 



EXTERNAL FORM 



CORNEA TRANSPARENT 



■>AJ 



TAOWLC rORU 



TICTM 



CIMB BUD 



DESCRIPTION OF STAGES* 

 STAGES I TO 17 (TABLE I) 



1. S^g at fertHlzatlon. 



2. Eetabllsbment of gray crescent area as first 



external evidence of development, sharply 

 defined at 1 hour. 

 5 to 6. Age given la time of appearance of 



cleavage furrov that eetabllahee the number 



of cells drawn for the stage. 

 7 to 9- Later stages In cell multiplication, 



best determined ty comparison of size of 



cells at vegetal pole. 



Appearance of dorsal lip. 



Blastopore approximately a semicircle. 



Complete blastopore (yolk-plug) stage. 

 13 • Silt blastopore or neural plate stage. 

 lU. Neural fold stage. 



Beginning of closure of neural folds, begin- 

 ning of elongation. Cilia begin to rotate 

 the embryo at about this stage. 



Closure of neural folds con^Jleted. 



Beginning of development of tall bud, marlced 

 off from body by ventral notch i^en embryo 

 Is viewed laterally. 



10. 

 U. 

 12. 



15 



* 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 com- 

 prised in a series of periods, each extending 

 from one stage until tile next. In practice the 

 points and the periods are not sharply dlatln- 

 gulshed and It la convenient to describe each 

 period in terms of th^ 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* 



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



