THE ORGANS OF THE OUTER GERM-LAYER. 



497 



By a continually daepening constriction (fig. 279 /) the lower 

 portion (S) is delimited from the utriculus (U), and finally 

 remains connected with it by a very narrow tubule only (canalis 

 utriculo-saccularis figs. 280 R and 282 2 ). Since the constriction 

 affects exactly that place of the labyrinth-sac from which the 

 recessus labyrinthi arises, the opening of the latter subsequently 

 comes to lie within the territory of the canalis utriculo-saccularis, at 

 about its middle (figs. 280 R and 282 2 ). In this manner there is 

 produced an appearance as though the recessus labyrinthi were split 

 at its beginning into two narrow tubules, one of which leads into the 

 sacculus, the other into the utriculus. 



By a second deep constriction (figs. 279, 280, 282) the sacculus 

 (S) is separated from the developing ductus cochlearis (dc\. Here 

 also a connection is 

 maintained by means 

 of an extraordinarily 

 fine connecting tubule 

 only (cr), which HENSEN 

 discovered and has de- 

 scribed as canalis re- 

 uniens. The ductus 

 cochlearis itself in- 

 creases greatly in 

 length, and at the same 

 time begins to be rolled 

 up in spiral turns in 

 the soft, enveloping, em- 

 bryonic connective tissue, until in Man it describes two and a half 

 turns (figs. 280 C and 282 Con). Since the first whorl is the 

 largest, and the others are successively narrower, it acquires a great 

 resemblance to a snail-shell. 



The alterations in the external form of the vesicle are accompanied 

 by changes in the nature of its epithelium also. This is separated 

 into the indifferent epithelial cells, which simply serve as a lining, 

 and the real auditory cells. The former are flattened, becoming 

 cubical or scale-like, and cover the greater part of the inner surface 

 of the semicircular canals, the sacculus, the utriculus, the recessus 

 labyrinthi, and the ductus cochlearis. The auditory cells, on the 

 contrary, are elongated, become cylindrical or spindle-shaped, and 

 acquire at the free surface hairs, which project into the endolymph. 

 By the separation of the vesicle into its various divisions the 



32 



Fig. 280. Diagram to illustrate the ultimate condition of 

 the membranous labyrinth [after WALDEVER]. 



U, Utriculus ; S, sacculus; Cr, canalis reunions ; M, recessus 

 labyrinthi ; C, cochlea ; K, blind sac of the cupola ; 

 V, vestibular blind sac of the ductus cochlearis. 



