( 267 ) 



TABLE XXXII. 



Systematic Survey of the Chief Stages in the Development 



of the Human Ear. 



I. First Stage. 

 The auditory nerve is an ordinary sensitive skin-nerve, which, during the 

 differentiation of the horn-plate, appears at a certain point on the skin of 



the head. 



II. Second Stage. 



The differentiated place of the horn-plate, at which the auditory nerve 

 appeared, forms a small special auditory groove in the skin, which has an 

 outer orifice in the appendage called the " labyrinth." 



III. Third Stage. 



The auditory groove has detached itself from the horn -lamina, and forms 

 a small closed auditory vesicle filled with fluid. The " labyrinth-appendage" 

 becomes rudimentary {AquwdAictus vestibuli) . 



IV. Fourth Stage. 

 The auditory vesicle differentiates into two connected parts, the ear- 

 pouch (utriculus) and the ear-sac (sacculus). Each of the two vesicles 

 receives a special main branch of the auditory nerve. 



V. Fifth Stage. 



Three semi-circular canals grow from the ear-pouch (as in all Amphi- 



rhina). 



VI. Sixth Stage. 



The "snail" (cochlea) grows from the ear-sac in Fishes and Amphibia; 



it is very insignificant, and is only developed as an independent part in the 



Amniota. 



VII. Seventh Stage. 



The first gill-opening (the blow-hole of Selachians) changes into the 

 tympanic cavity and the Eustachian tube ; the former is externally closed 

 by the tympanic membrane (Amphibia). 



VIII. Eighth Stage. 



The small bones of the ear (ossicula auditus) (the hammer (malleus) and 

 anvil (incus) from the first gill-arch, the stirrup (stapes) from the second) 

 develop from parts of the first and second gill-arches. 



IX. Ninth Stage. 

 The external ear is developed, together with the bony ear-canal. The 

 shell of the ear is pointed and movable (as in most lower Mammals). 



X. Tenth Stage. 

 The ear-shell, with its muscles, becomes disused and a rudimentary 

 organ. It is no longer pointed, but, on the contrary, has a curved rim with 

 a small ear-flap (as in Anthropoid Apes and Men) . 



