THE SENSE-ORGANS 487 



The otic vesicle itself develops, mainly by the unequal 

 growth of the different regions, into the labyrinth. This de- 

 velops first into two expanded portions, utriculus and sacculus, 

 with a restricted portion between them, the utriculo-saccular 

 canal. [Cf. Figs. 133 and 134.] From the utriculus, which 

 lies dorsally, develop three flattened folds, which by the adhe- 

 sion and subsequent atrophy of the middle portion of their 

 walls, develop their marginal portions into tubes. Thus are 

 formed the three semicircular canals, which are constant in 

 all vertebrates above the cyclostomes, and vary but little in 

 general appearance or relationships. They are set approxi- 

 mately at right angles to one another in such a way that one 

 lies horizontally and the other two vertically, but at an angle 

 of about 45 with the bilateral plane of the body. They 

 empty at either end into the utriculus, one end of each being 

 expanded into a flask-shaped ampulla. These are so placed that 

 the ampullae of the anterior vertical and horizontal canals open 

 together into a pocket of the utriculus (recessus utriculi), 

 while that of the posterior vertical canal opens by itself into a 

 similar pocket on the other side (sinus utriculi posterior). 

 The two vertical canals unite at their expanded end and enter 

 or form the sinus utriculi superior, a diverticulum which ex- 

 tends directly upward; the unexpanded end of the horizontal 

 canal enters the main body of the utriculus unassociated. 



The parts of the utriculus with its derivations, the semicir- 

 cular canals, appear first in the form described above in the 



extends posteriorly, dorsal to the spinal cord and outside of the dura 

 mater, as far as the tail rudiment (urostyle). This extensive sinus sends 

 to the roots of the spinal nerves a series of diverticula which wrap them- 

 selves about the spinal ganglia and expand into sacs containing granules 

 of calcium carbonate. In many reptiles the duct reaches the top of the 

 skull and even escapes, its terminal sac being subcutaneous. In snakes 

 this sac contains calcareous crystals, which in some cases may be seen 

 through the skin of the living animal. This apparatus reaches its highest 

 development, so far as reptiles are concerned, in the lacertilian family of 

 the geckos (Ascalabota) , where the sac escapes from the skull through 

 the parieto-occipital suture and pushes its way between the muscles of the 

 neck and shoulder as far as the pharynx. It is filled with a soft cal- 

 careous mass. 



