THE LATER DEVELOPMENT OF THE CHICK 325 



Meanwhile the external auditory meatus forms, as a depression 

 on the surface of the head between the dorsal and ventral 

 portions of the original hyomandibular pouch. This depres- 

 sion deepens and finally the tympanic cavity meets it; the 

 membrane separating the two cavities does not perforate, but 

 remains as the tympanic membrane or- ear-drum. In the in- 

 terior of this membrane, the mesenchyme which remains be- 

 tween its outer ectodermal and inner endodermal layers, dif- 

 ferentiates into the stapes, which comes into relation with the 

 malleus and incus, already marked out in the mesenchymatous 

 rod crossing the tympanic cavity. 



3. The Olfactory Organ 



This develops much later than the eye and ear, a fact which 

 is possibly correlated with the secondary importance of the 

 olfactory sense in the birds. Its appearance is made at the 

 close of the second day, as a pair of circular thickenings in 

 the superficial ectoderm just anterior to the eyes. These 

 thickenings are due to the elongation of the epithelial cells and 

 mark the primary differentiation of the olfactory epithelium. 

 Each patch soon becomes invaginated, forming an olfactory 

 pit, which remains open to the surface of the head. Out- 

 growth of the fore-brain produces an apparent shifting of the 

 external openings of the olfactory pits to the ventro-lateral 

 surfaces of the head in the margin of the stomodaBum. The 

 two pits are separated by a broad ridge, produced by the en- 

 largement of the fore- brain; this is the fronto-nasal process. 

 The outer border of the opening of the olfactory pit also 

 becomes elevated, and about the fifth day becomes joined 

 with the fronto-nasal process by a bridge of tissue, extending 

 across the olfactory opening and dividing it into upper and lower 

 portions. This bridge enlarges as the rudiment of a part of 

 the upper jaw, and the two openings thus have quite different 

 fates. The upper is carried outward and upward and forms 

 the external nares; the lower is carried downward and inward 

 (relatively) as the internal nares (choance). 



