DEVELOPMENT OF PERIOTIC TISSUE SPACES 319 



invest the glossopharyngeal nerve and extend down along its 

 trunk and pass closely and directly posterior to the region of 

 the fenestra cochleae (rotundum). A thin-walled tubular 

 pouch projects from these spaces leaving the nerve trunk and ex- 

 tending obliquely toward the scala tympani in a direction that 

 would meet it just distal to the fenestral impression on its basal 

 surface. This fundament of the aquae ductus cochleae is present 

 in fetuses 85 mm. CR length, but is longer in the 130 mm. fetus 

 where it nearly reaches the scala tympani. The communica- 

 tion must be established soon after this. 



SUMMARY 



The earliest histological evidence of the formation of the peri- 

 oticular spaces occurs near the stapes, in the reticulum lying be- 

 tween the sacculus and the fenestra vestibuli, where in embryos 

 between 30 mm. and 40 mm. long it can be seen that its meshes 

 are becoming irregular and its spaces are beginning to coalesce. 

 This constitutes the rudiment of the vestibular cistern. It makes 

 its appearance before there is any trace of the scalae, but it is 

 not until the fetus becomes about 50 mm. long that the cistern 

 is definitely outlined and clearly differentiated from the adjoin- 

 ing reticulum. 



After the cistern, the scala tympani is the next space to be- 

 come established. It can be recognized as a moderate widening 

 of the meshes of the reticulum in the region of the fenestra coch- 

 leae in fetuses 43 mm. long, along the basal border of the first 

 turn of the cochlear duct. The scala vestibuli, as can be seen in 

 fetuses 50 mm. long, develops as an extension downward of the 

 cistern along the apical border of the cochlear duct. Starting 

 from these definite foci these three spaces spread into their des- 

 tined territory absorbing as they go the enlarging reticular spaces 

 of the invaded region by a process of space-coalescence. In 

 fetuses 85 mm. long the two scalae extend downward along the 

 cochlear duct to its last turn as two separate spaces which do not 

 communicate with each other. When they reach the tip of the 

 duct, which occurs in fetuses about 130 mm., crown-rump length, 



