262 



THE OLD MOUTH AND THE NEW. 



165, 167, a.d.o. In adult cyclostomes in place of these organs, there are three 

 pairs of tentacle-like appendages, belonging apparently to the premaxillary, 

 maxillary, and mandibular arches (Fig. 175); and in embryo sturgeons, there 



162 



J 163 



b.riv. 



Figs. 162 and 163. — Frog embryos, seen from the side, showing the beginning of the external gills, or cephalic appen- 

 dages, operculum, cranial ganglia, and oral arches. R. septemtrionalis. 



?P e>c.g 



Figs. 164, 165 and 166. — Young tadpole of R. septemtrionalis. Fig. 164, — Tadpole from the side, showing 

 early stage in the formation of the peribranchial chamber. Pig. 165. — Same; older stage; gills completely en- 

 closed in peribranchial chamber. Fig. 166. — Oral region, from in front. 



appears to be a pair of similar structures on the premaxillary and one on the 

 maxillary arch. (Fig. 174.) In Polypterus, Budgett has described a "cement 

 organ," similar to the mandibular glands of the frog, located at first in front of 



