178 THE GERM LAYER DERIVATIVES 



cased in mesenchyme which gives rise to cartilage that is finally 

 displaced by bone. This makes up the auditory capsule which is 

 mesodermal. Between the capsule and the inner ear (membranous 

 labyrinth) is the perilymph space which is filled with the perilymph 

 fluid, derived from mesenchyme. 



The Middle Ear. This portion of the ear grows as an endo- 

 dermally lined tube extending dorso-laterally from the original hyo- 

 mandibular pouch and developing a terminal chamber. The chamber 

 comes to lie between the inner ear and the head ectoderm. That por- 

 tion of the endoderm in contact with the head ectoderm (original hyo- 

 mandibular plate) becomes the tympanic membrane (ear drum), 

 later to be invaded by mesenchyme which forms blood vessels and 

 connective tissue. This membrane is supported by a cartilaginous ring 

 known as the annulus tympanicus (tympanic ring) which is believed 

 to arise from the palatoquadrate bone. These latter two structures are 

 presumed to cause the development of the tympanic membrane by 

 induction. 



This endodermally lined chamber expands inwardly to make con- 

 tact with the inner ear capsule at the site of the foramen ovale, an 

 aperture in the inner ear capsule and extending through the perilymph 

 space. This aperture is obstructed by a cartilaginous plug known as the 

 operculum. This operculum later becomes ossified into bone. A 

 cartilaginous rod arises from the roof (dorsal wall) of this cavity and 

 extends from the operculum of the inner ear to the tympanic mem- 

 brane and across the middle ear cavity, and is known as the columella 

 or plectrum. This columella is mesodermal, arising from the inner 

 stapedial plate (part of the otic capsule) and a cartilage from the 

 palatoquadrate bar. The columella becomes partially ossified and con- 

 veys to the inner ear the auditory vibrations which impinge upon the 

 outer tympanic membrane. It is derived presumably from part of the 

 second or the hyoid arch. 



The original connection of the pharynx and the middle ear cavity, 

 the dorso-lateral tubular growth from the hyomandibular pouch, re- 

 mains in the adult as the tubo-tympanic cavity or the Eustachian tube 

 and is lined with endoderm. 



The Olfactory Organs. 



The external olfactory placodes arise as thickenings in the neural 

 ectoderm of the sense plate, dorso-lateral to the stomodeum, at about 



