THE LATER DEVELOPMENT OF THE FROG 153 



the tympanic cavity. This is the last part of the auditory 

 apparatus to be formed. Its inner end is formed as a cartilage 

 (the operculum) appearing in the tissue plugging the large 

 foramen ovale in the outer side of the cartilaginous auditory 

 capsule. From this region another cartilage, the plectrum, 

 differentiates, toward the close of metamorphosis, along the 

 dorsal wall of the tympanic cavity, finally extending to the 

 tympanic membrane where it connects with a ring-like cartilage 

 developed from the palato-quadrate cartilage (see below). The 

 elements of the columella are thus not genetically related with 

 the visceral skeleton (palato-quadrate cartilage). While the 

 opercular part of the columella is becoming chondrified, the 

 tympanic cavity extends dorsally and finally surrounds it 

 completely, leaving it in its definitive position crossing the 

 tympanic cavity. 



3. Tlte Olfactory Organ 



The olfactory organs appear very early, before the brain 

 closes, as a pair of ectodermal thickenings either side of the 

 head, above and anterior to the future mouth region. Only 

 the deeper nervous layer of the ectoderm is involved in the 

 thickening, and the superficial layer disappears here, leaving 

 only its pigment scattered among the remaining cells. These 

 thickenings are immediately anterior to the lens placodes 

 and may be termed the olfactory placodes (Fig. 52, A): these 

 appear to have no relation to the median anterior ectodermal 

 thickening which is present at this time. These placodes 

 soon fold inward forming simple olf actor y^joits, the rudiments 

 of the true olfactory cavities, their walls becoming the olfactory 

 epithelium (Fig. 52, B). A few cells detach from the inner 

 surface of the olfactory placode and mingle with cells derived 

 from the surface of the telencephalon : these appear to be 

 equivalent to the crest ganglion, and from them are formed, 

 apparently, the sheath cells of the true olfactory nerve fibers, 

 which are outgrowths of the sensory cells of the olfactory 

 placode. 



