FORMATION OF STAPEDIAL PLATE 93 



in neighboring sections, to pass under the internal carotid. 

 Mesial to the latter is the large internal jugular. Lateral to 

 the carotid is the cartilaginous quadrate. Ventral to it is the 

 parachordal cartilage which surrounds the notochord and is 

 continuous laterally with the otic capsule. Ventral to the hind 

 brain is the parachordal cartilage which surrounds the notochord 

 and is continuous laterally with the otic capsule. 



Figure 3 represents a frontal section through the operated side 

 of the same embryo. This side presents striking contrast to the 

 conditions just described for the normal side, inasmuch as cer- 

 tain structures are absent. The auditory epithelium has been 

 removed. The otic capsule has failed to develop. The colu- 

 mella, similar to that of the normal side, projects mesially from 

 the external auditory meatus. There is no stapedial plate. The 

 inner end of the columella abuts against (but is not fused with) 

 the parachordal cartilage. Some of the region which would 

 normally have been occupied by the otic capsule has been in- 

 vaded by the Gasserian ganglion, which seems to have migrated 

 postero-ventrally and completely imbibed or fused with the 

 facial ganglion. Thus it happens that in neighboring sections 

 the large Gasserian ganglion appears to give rise to the facial 

 nerve, which sends a branch to the anlage of the stapedius 

 muscle or at least of a muscle in close relation to the columella. 

 In most other respects the conditions are similar to those on the 

 normal side. 



Conditions similar to those in this embryo have been obtained 

 in a large number of cases. It seems highly probable that the 

 stapedial plate owes its formation to the same general stimulus 

 which initiates the cartilage-formation that constitutes the otic 



Figs. 2 and 3 Frontal sections of the otic regions of an eight-day chick from 

 which the right otocyst was removed at a time at which the embryo was forty- 

 five hours old. Figure 2 shows the normal side while figure 3 shows the oper- 

 ated side in which a stapedial plate has failed to develop. P. E. C. No. 1117. 

 Ac, acoustic nerve; Col., columella; E.a.m., external auditory meatus; Ephr., 

 epibranchial cartilage; Fac, facial nerve; G.g., trigeminal ganglion; Gl., glosso- 

 pharyngeal nerve; Int. cor., internal carotid; Int. j., internal jugular; Nch., noto- 

 chord; Ot. cp., otic capsule; Otcst., otocyst; Par., parachordal; Qudr., quadrate; 

 Rh., hind brain; Sq., squamosal; St. p., stapedial plate. 



