THE ORAL ARCHES. 



259 



unite, obliterating a part of the median groove. Between the anterior margins 

 of the premaxillary lobes a small pit is left that gradually deepens, forming the 

 anlage of the hypophysis, D, E, F, hyp. 



The prominent maxillary lobes move laterally and forward and unite with the 

 premaxilke, although they are still distinctly marked off from them by the second 



op w 



md 



,ado 



arl.o 



C 



FIG. 160. Frog embryos seen from the anterior end, showing the concrescence of the oral metamers in front, and on 



the haemal side of the fore brain. Rana septemtrionalis. 





F 





FIG. 161. Frog embryos, seen from the anterior haemal sufrace of the head, showing successive stages in the 

 concrescence of the three pairs of oral arches, the pre-maxillse, maxillae, and mandibles, and their relation to the 

 apex of the forebrain, to the mouth, hypophysis and sucking discs. R. septemtrionalis. 



transverse groove, E and F, mx. This groove finally extends past the olfactory 

 pits toward the eyes, probably initiating the formation of the tear duct. The 

 mandibular lobes, meantime, have become very prominent. Later they unite in 

 the median line to form the lower jaw, E and F, md. 



