DEVELOPMENT OF THE SENSE ORGANS 391 



The external auditory meatus is formed as an ingrowth of the first 

 branchial groove. In embryos of 1 2 to 1 5 mm. the wall of this groove is in 

 contact dorsally with the entoderm of the first pharyngeal pouch. Later, 

 however, this contact is lost, and, during the latter part of the second 

 month according to Hammar, an ingrowth takes place from the ventral 

 portion of the groove to form a funnel-shaped canal. 



The lumen of this tube is temporarily closed during the fourth and fifth months, but 

 later re-opens. During the third month a cellular plate at the -extremity of the primary 

 auditory meatus grows in and reaches the outer end of the tympanic cavity. During the 

 seventh month a space is formed by the splitting of this plate and the secondary, inner por- 

 tion of the external meatus is thus developed. 



The tympanic membrane is formed by a thinning out of the mesodermal 

 tissue in the region where the wall of the external auditory meatus abuts 

 upon the wall of the tympanic cavity. Hence it is covered externally 

 by ectodermal and internally by entodermal epithelium. 







