234 



GASTRULA-ARRESTED EMBRYOS 



ST 



NQ 



18* 



EXTERNAL 

 FORM 



ST 



NQ 



AGE 

 HRS 

 18* 



EXTERNAL 

 FORM 



ST^ 

 NO 



HRS 

 18* 



EXTERNAL 

 FORM 







13 



36 



2-5 



3+ 



4-5 



5f 



12 



16 



10 



19 



24 



12 



28 



14 



40 



45 



50 



17 



58 



Fig. 2. Normal stages in the development of embryos in the species 

 Rana sylvatica. (From PoUister and Moore, 1937.) 



