180 



THE GERM LAYER DERIVATIVES 



Neural ectoderm 



Superficio 

 ectoderm 



Olfactory 

 pit (externa 

 nares) 



Olfactory 

 placode 



Olfactory 

 nerve (I) 



Head 

 mesenchyme 



Lateral 

 appendix 



Choanal 

 canal 



Internal nares (choano) 



Pharynx 



Development of the olfactory organ of the frog. {Top, left) Sagittal section 

 through the olfactory placode and nerve. {Top, right) Posterior transverse 

 section through the choanal canal. {Bottom) Schematic reconstruction of the 

 embryonic olfactory organ. 



tip of the tail. Along these cords arise groups of sensory cells which 

 grow through the epidermis to become exposed along the sides of the 

 body as the lateral line system. The exposed cells are ciliated, and are 

 therefore sensitive to vibrations in the surrounding aquatic medium. 

 They are protected by inner and outer sheath cells and a basement 

 membrane and are connected functionally with branches of the lateral 

 line nerve. At the head level the extensions of this system seem to be 

 innervated by cranial nerves VII, IX, and X, principally the latter. 

 This structure is no doubt a vestige of the aquatic ancestry of the 

 Anura, for, as in lower forms, these organs are innervated by a branch 

 of the vagus (X) ganglion, known as the lateral line nerve or ramus 

 lateralis. In the Anura this system disappears by the time of meta- 

 morphosis. 



