198 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



develops an area of sensory epithelium like that of the lateral-line 

 system, this stage may be compared to an isolated canal organ with a 

 single pore. 



In the amphibia and some of the ganoids, where there is a two-layered ecto- 

 derm from the early stages, there is never an open auditory cup. The lower, 

 so-called nervous layer of the ectoderm is alone concerned in the formation 

 of the auditory vesicle, while the outer layer extends as an unbroken sheet 

 across the cup. In the elasmobranchs the endolymph ducts open to the ex- 

 terior throughout life, the external pores being recognizable on the top of the 

 head. Elsewhere they later lose their external openings, and the distal end of 

 each usually expands into an enlargement, the sacculus endolymphaticus, 

 but in the amphibia the ducts of the two sides may unite dorsal to the brain, 



Fig. 205. — Diagram of developing human labyrinth from 6 to 30 mm. long, after 

 Streeter. am, ampulla; c, cochlear region and cochlea; au, ampuUo-utricular region; d, 

 endolymph duct; e, endolymph region; sc, semicircular canal; se, endolymph sac; s, 

 sacculus; u, utriculus; us, utriculo-saccular canal; v, vestibule. 



while other parts may branch and grow in a root-like manner, in the canal of the 

 spinal cord, sending diverticula (frog) into the so-called calcareous glands, which 

 surround the basal parts of the spinal nerves. 



The next stage in the auditory vesicle is its differentiation by a 

 constriction into two chambers, an upper vestibuliim or utriculus and 

 a lower sacculus (fig. 205), the two connected by a narrow sacculo- 

 utricular canal. The sensory area becomes divided between the two. 

 The anterior, posterior and lateral walls of the utriculus now produce 

 flattened outgrowths, the lateral in the horizontal, the others in ver- 

 tical planes, and parts of the sensory areas extend into each. Next, 

 the walls of these diverticula become pinched together so that each 

 pocket is converted into a tube or canal, open at either end into the 



