126 EMBEYOLOGY OF THE LOWES VEETEBEATES oh. 



anterior and posterior naris : the morphologically posterior naris 

 coming to lie in front of that which was originally anterior. 



In the Amphibian and Amniote the upper lip, which completes 

 the buccal cavity in front, is situated between the anterior and 

 posterior narial openings, so that the latter opens into the buccal cavity 



Fig. 69.— Ventral views of head region of larva of Procerus at stages 31 (A), 32 (B), 34 

 (C), 34 (D), 35 (B), and 36 - (P), to illustrate the development of the olfactory organ. 



c.o, cement organ; e.g, external gill; olf, olfactory organ ; olfl, anterior ("external") naris; oZ/2, 

 posterior ("internal ") naris. In C the curved line running across the ventral side of the head is the 

 posterior margin of the mouth : the darkly shaded grooves passing inwards and forwards from its 

 outer ends are the olfactory rudiments. 



(internal naris), while the former remains outside (external naris). 

 In the developing Amniote embryo (cf. Fowl, Chap. X.) the o-eneral 

 arrangements, while essentially the same as those of Protopterus, are 

 somewhat obscured by the modelling of the face region. The ridge 

 which forms the upper lip, or anterior boundary of the buccal cavity, 

 is cut across by the olfactory slit, here a wide and deep cleft, into a 



