194 Vertebrate Embryology 



the lining of the cleft ; and a middle layer 

 of mesoblast, from the uncleft mesoblast of 

 the neck region. During a greater part of 

 foetal life the tympanic membrane is very 

 thick, and bears little resemblance to the 

 structure in the adult. 



The changes above described, like many 

 others given in even so brief an account as 

 this, can be made out only with great diffi- 

 culty, so that the student who is just begin- 

 ning the study of embryology will generally 

 have to take such statements for granted. 



The fate of the visceral folds should, per- 

 haps, be discussed in connection with the 

 development of the skeleton, but it will be 

 convenient to take up the discussion at this 

 point, along with that of the visceral clefts, 

 with which they are so intimately associated. 



The last two arches, the so-called second 

 and third arches or folds, apparently entirely 

 disappear and leave no trace in the adult. 



Parts of the two arches in front of these, 

 that is, the first visceral and the hyoid, be- 

 come converted into the hyoid apparatus of 

 the adult. 



The first arch, the mandibular, is the most 

 important, in regard to the adult structures 



