GILL- PLATE AREA 

 NASAL PIT 



Fig. 226. External views of embryos of Rana sylvatica and Rana pipiens. (A to J 

 after Pollister and Moore: Anat. Rec, 68; K and L after Shumway: Anat. Rec, 78.) 

 (A, B) Lateral and ventral views of 5-mm. stage. Muscular movement is evident at this 

 stage, expressed by simple unilateral flexure; tail is about one-fifth body length. (Pollister 

 and Moore, stage 18.) (C, D) Lateral and ventral views of 6-mm. stage. Primitive 

 heart has developed and begins to beat; tail equals one-third length of body. (Pollister 

 and Moore, stage 19.) (E, F) Similar views of 7-mm. stage. Gill circulation is established; 

 hatches; swims; tail equals one-half length of body. (Pollister and Moore, stage 20.) 

 (G, H) Ten-mm. stage, lateral and dorsal views. Gills elongate; tail fin is well developed 

 and circulation is established within; trunk is asymmetrical coincident with posterior 

 bend in the gut tube; cornea of eyes is transparent; epidermis is becoming transparent. 

 (Pollister and Moore, stage 22.) (l, J) Eleven-mm. stage, true tadpole shape. Oper- 

 cular fold is beginning to develop and gradually growing back over gills. (K, L) 

 Eleven-mm. stage of R. pipiens embryo. Observe that opercular folds have grown back 

 over external gills and developing limb buds; opercular chamber opens on left side of 

 body only. Indicated in fig. 257B. 



470 



