7O MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



the other two are accessory in character. The sensory portion 

 of the inner ear arises from the ectoderm. At first it is a cup- 

 like depression on either side of the 

 head. Then it sinks deeper and its 

 edges unite, converting the cup into a 

 closed sac, 1 the primitive otic vesicle. 

 In all forms the closure lags at one 

 point, and in this way, by the in sinking 

 of the rest, a slender tube, ductus en- 

 dolymphaticus or aqueductus vestibuli, 

 is formed, reaching to the parent ecto- 

 derm ; and in the elasmobranchs this 

 tube opens throughout life to the ex- 

 terior by a small opening near the 

 middle line of the top of the head. 

 FIG. 73. Involution of audi- In other forms it becomes closed, and 



tory ^ epithelium, A, to form the in Some groups the ducts of the two 

 auditory vesicle in the embryo gides CO nnect above the brain, 



tern. CJ3, cerebellum ; G, audi- ' 



tory ganglion ; A; notochord. Distally each duct expands into a sac- 



cus endolymphaticus, which in the 

 lamprey, according to Ayers, is sensory. 



The otic vesicle is at first spherical, or oval, but it soon divides 

 by constriction into an upper portion, the utriculus, and a lower, 

 sacculus, connected by a narrower utriculo-saccular canal. 

 Flattened outgrowths arise from the walls of the utriculus, the 

 walls of which become pinched together so that each outgrowth 

 becomes converted into a semicircular canal, opening at either 

 end into the utriculus. In the myxinoids there is but one of 

 these canals ; the lampreys have two, and all other vertebrates 

 three. Two are in vertical planes at nearly right angles to each 

 other, and from their position are known as the anterior and 

 posterior canals ; the third is horizontal in position, and is 

 called the external canal. Each canal bears an enlargement, 

 the ampulla, at one end , 2 at the anterior end of the horizontal 

 canal, at the ventral ends of the vertical canals. 



1 In some forms (e.g., amphibia) only the deeper layer of the ectoderm participates in 

 the formation of the otic vesicle. Otherwise the history is much the same. 



2 The single semicircular canal of the myxinoids has an ampulla at either end. 



