208 Henry Fox 



teriorh' with the lateral margin (vestibular fold) of the oral cavity. 

 Subsequently this connection is interrupted and the submeckelian fold 

 then grovrs out as a prominent shelf-like protuberance underlying 

 Meckel's cartilage. 



Period III. The transformation of the primary tympanic pouch into 

 the definitive tympanic pouch and Eustachian tube is marked by the 

 following features : 



The peripheral jjortion of the j)0uch becomes relatively fixed in posi- 

 tion by the segregation of Meckel's cartilage with its manubrial process 

 and the auditory capsule. 



The basal j)ortion, on the other hand, continues to be carried down by 

 the downgrowth of the alveolo-lingual fold. 



The combined effect of these two processes is to give the pouch a 

 peripherally ascending course. 



An incision forms at the postero-internal angle of the pouch between 

 it and the dorsum of the second pouch. This incision rapidly extends 

 forwards as an ever-widening cleft between the base of the pouch and 

 the wall of the pharj^nx. 



In consequence of this process the connecting part of the pouch is 

 progressively constricted until it forms a narrow tube, the Eustachian 

 tube. 



The remainder of the pouch retains its original wide extent and forms 

 the tympanic pouch. 



The later changes relate mainly to modifications in the detailed 

 structure of the tympanic pouch. Among them are the formation of 

 the manubrial fossa, the reduction of the submeckelian fold and the 

 formation of the tympanic membrane. 



The manubrial fossa lodges the ventral extremity of the manubrium. 

 At first it is a shallow impression on the lateral surface immediately 

 underlying the posterior part of the submeckelian fold. With the 

 formation of the definitive t3^mpanic pouch it rapidly deepens to form 

 a cup-like depression. Subsequently this elongates at its ventral ex- 

 tremity to form the fissure-like groove characteristic of its final ^fage. 



The submeckelian fold is at first very prominent and partly encloses 

 a Meckelian fossa. The latter later assumes a more flattened form and 

 the fold at the same time liroadens until it is al)sorbed into the wall of 

 the pouch. In the latest stage the submeckelian fold forms only 'an 

 inconspicuous swelling on the outside of the tubo-tympanal border. 



The tympanic membrane is formed by the progressive approximation of 



