THE ORGANS OF THE OUTER GERM-LAYER. 503 



onic cells (ysp), and for the blood-vessels, and constitutes a connective- 

 tissue plate which is subsequently ossified to form the lamina spiralis 

 ossea. Thirdly, it clothes with a thin layer the epithelial ductus, 

 serving for the distribution of the blood-vessels on the latter, and 

 together with it is designated as the membranous ductus cochlearis. 

 Fourthly, it lines the inner surface of the cartilaginous capsule as 

 perichondrium (P). Finally, fifthly, there is formed a connective 

 tissue plate (Y) extending between the cartilaginous ridge which, 

 as previously described, projects inward from the capsule and the 

 connective -tissue axis of the cochlea (M). It is stretched out between 

 and separates the successive turns of the membranous cochlear duct, 

 so that the latter now comes to lie in a large canal, the wall of which 

 is in part cartilaginous, in part membranous. This canal is the 

 foundation of the bony cochlea. 



That portion of the mesenchyme which is not converted into 

 fibrous connective tissue becomes gelatinous tissue (g and </). It 

 forms bet\veen the parts just mentioned two spiral tracts, one of 

 \vhich is located above and the other below the membranous ductus 

 cochlearis and the membranous lamina spiralis. The tracts there- 

 fore occupy the place of the scala vestibuli (SV) and the scala 

 tympani (>ST). The latter arise, even before the process of ossifica- 

 tion begins, in exactly the same w r ay as the perilymphatic spaces 

 in the case of the semicircular canals and the vestibule. In the 

 gelatinous tissue the matrix becomes softer and more fluid, and 

 the cells begin to undergo fatty degeneration. Small fluid-filled 

 cavities make their appearance ; these become joined to one another, 

 and finally the \vhole space occupied by gelatinous tissue is filled 

 with perilymph. The process of softening begins at the base of the 

 cochlea in the region of the first spiral (ST and SV), and advances 

 slowly toward the cupola. Here vestibular and tympanic scalse finally 

 unite, after the last remnant of the gelatinous tissue has been dis- 

 solved. Figure 283 exhibits a stage in which, at the base of the 

 cochlea, the perilymphatic spaces (SV and >ST) have been formed, 

 and only small remnants of the gelatinous tissue ((/') are present, 

 whereas at the apex of the cochlea the process of liquefaction of 

 the gelatinous tissue (y) has not yet taken place. 



With the development of the scala? the membranous ductus 

 cochlearis changes form. Whereas its cross section was formerly 

 oval, it now assumes the form of a triangle (dc). For those portions 

 of the wall which are adjacent to the vestibular and tympanic scalse, 

 and which have been named from them, gradually become flattened, 



