Development of the Third Day 193 



that, as far as can be seen in surface views, 

 they disappear. The first, second, and third 

 clefts do, apparently, completely disappear 

 and leave no trace in the adult ; but the 

 most anterior cleft, the hyomandibular, 

 although, like the rest, closing at the outer 

 end, does not close throughout its length, 

 and retains its connection with the cavity of 

 the pharynx. The exact changes that now 

 take place are somewhat in dispute, but it 

 seems reasonably certain that the inner, un- 

 closed portion of the cleft forms an enlarge- 

 ment, at its outer end, which becomes the 

 tympanic cavity, while the rest of the cleft 

 persists as the Eustachian tube. 



The external auditory meatus is built up 

 as a short tube on the outside of the head, 

 opposite the position of the tympanic cavity; 

 it may be partially formed by a slight de- 

 pression of the surface ectoblast. The layer 

 of tissue, formed by the closure of the outer 

 end of the hyomandibular cleft, which lies 

 between the tympanic cavity and the external 

 auditory meatus, becomes the tympanic mem- 

 brane. It is evidently composed of three 

 parts ; an external layer of ectoblast, from the 

 surface ; an inner layer of entoblast, from 



