THE ORGANS OF THE OUTER GEUM-LAYER. 511 



7. The tympanic membrane, which at first is rather thick and 

 only gradually becomes reduced to a thin, tense membrane, is de- 

 veloped out of the closing plate of the first visceral cleft and the 

 adjacent parts of the visceral arches. 



8. The tympanic cavity and the Eustachian tube are developed 

 out of a depression on the median side of the tympanic membrane, 

 the sulcus tubo-tympanicus, and out of an evagination from it 

 extending upward, outward, and backward. 



9. The tympanic cavity is at first extremely small, the connective 

 tissue of the mucous membrane that surrounds it being gelatinous 

 [and voluminous]. 



10. The auditory ossicles and the chorda tympani lie at first 

 outside the tympanic cavity in the gelatinous tissue of its wall ; it is 

 only after shrivelling of the gelatinous tissue that they come to lie 

 in folds of the mucous membrane, which project into the now more 

 capacious tympanic cavity (incus-fold, malleus-fold). 



1 1 . The external meatus is developed from the periphery of the 

 depression that lies on the lateral side of the tympanic membrane ; 

 the aiiricle arises from six elevations, which are converted into 

 tragus, antitragus, helix, antihelix, and the lobule of the ear. 



C. The Development of the Organ of Smell. 



The organ of smell is, like the eye and ear, a product of the outer 

 germ-layer, from which it is developed somewhat later than the two 

 higher sensory organs. It first becomes noticeable, at either side 

 of the broad frontal process (fig. 274) previously described, as a 

 thickening of the outer germ-layer which His has designated in 

 human embryos as nasal area. Both fundaments soon become more 

 distinct owing to the fact that each nasal area becomes depressed 

 into a kind of trough, the edges of which rise up as folds (fig. 286). 

 An olfactory lobe, which has been formed meantime by an evagina- 

 tion of the cerebral vesicle, grows out on either side to the thick- 

 ened epithelium of this area, where its nerve-fibrillie terminate. 



The two olfactory pits, which are formed in a similar manner in 

 all Vertebrates with the exception of the Cyclostomes, in which only 

 an unpaired pit arises, are separated from each other by a consider- 

 able distance. They therefore appear at first as distinctly paired 

 structures, whereas in their ultimate condition in the higher 

 Vertebrates they have approached each other toward the median 

 plane and become an apparently unpaired organ, the nose. 



