FORMATION OF STAPEDIAL PLATE 91 



typically developed otocyst, around which the mesenchyme has 

 begun to condense, staining rather deeply. On the operated 

 side of the same embryo (right side of figure 1) it will be noticed 

 that the otocyst is entirely lacking. The mesenchyme occupy- 

 ing the former region of the otocyst resembles in every way the 

 other surrounding, lightly staining mesenchyme, and shows no 

 evidence of condensation into an otic capsule. There was 

 no attempt at regeneration of the removed auditory epithelium. 

 This embryo had not yet developed visceral cartilages. 



We may now consider cases in which the embryos which had 

 been operated upon were allowed in each case to live until the 

 stapes of the normal side was well developed. Eight and nine- 

 day embryos were found to be most favorable for study. 



Figure 2 represents a frontal section through the otic region 

 on the normal side of an eight-day embryo from which the right 

 otocyst was almost completely removed at the forty-fifth hour 

 of incubation. The columella is seen lying between the external 

 auditory meatus and the fenestra ovalis. Fused to the inner end 

 of the columella is a flange-like ring of cartilage; but the two are 

 everywhere separated by a distinct perichondrium which seems 

 to be shared in common by the periphery of the mesial end of the 

 columella and by the lateral internal surface of this flange of 

 cartilage, the stapedial plate. A large part of the stapedial 

 plate, or more correctly, the stapedial flange — projects freely 

 into the mesenchyme. A portion of it, however, is continuous 

 with the cartilage of the otic capsule, and seems always to have 

 been a part of that cartilage. Situated inside the otic capsule 

 is the auditory epithelium. The plane of section lies transverse 

 to the transitional portion of the epithelium between sacculus 

 and lagena. Mesial and dorsal to this epithelium is a portion 

 of the acoustic nerve. A small branch of the internal jugular 

 vein projects into the otic capsule. Dorsal to the capsular 

 region, the facial nerve is seen emerging from its foramen. The 

 plane of section is too far ventral to show the proximal connection 

 of this nerve. A section in such a plane is shown in another 

 embryo in figure 8. Dorso-anterior to the facial nerve will be 

 noticed the large Gasserian ganglion, fibers from which are seen, 



