300 THE DEVELOPMENT OF THE CHICK 



During the seventh and eighth days the enlarging cartilaginous 

 labyrinth presses down on the Eustachian tube and hinders its further 

 enlargement. On the eighth day the tube is a wide but narrow slit 

 which appears crescentic in a sagittal section of the head (Fig. 150). 



Some rather obscure details about the formation of the tubo-tym- 

 panic canal are mentioned here as suggestions for further work on the 

 subject. On the sixth day almost the entire roof is composed of flat- 

 tened cells similar to the roof of the pharynx ; the floor, however, is lined 

 with a columnar epithelium which extends out to and surrounds the 

 distal extremity; it seems probable that this terminal chamber lined 

 on all sides by columnar epithelium represents the first visceral pouch 

 proper. On the eighth day the cavity of this distal chamber is com- 

 pletely constricted off from the main tympanic cavity, though it is still 

 connected with the latter by a solid rod of cells, which gives unequivocal 

 evidence of its origin. I do not know what becomes of this separated 

 cavity later. (See Fig. 168 X.) 



(6) The External Auditory Meatus and the Tympanum. We 

 have already seen that on the ectodermal side there are originally 

 two depressions corresponding to the first visceral pouch, viz., 

 a dorsal round one in which a temporary perforation is formed, 

 and an elongated ventral furrow. Between these is a bridge of 

 tissue within which the external auditory meatus arises as a new 

 depression, first clearly visible on the sixth day, when it is sur- 

 rounded by four slight elevations, two on the mandibular and 

 two on the hyoid arch. The meatus gradually becomes deeper 

 and tubular, mainly owing, I think, to the elevation of the sur- 

 rounding tissue, the bottom of the meatus, or tympanic plate, 

 being held in position by the forming stapes. The meatus is 

 directed in a general median direction with a slight slant dorsally 

 and posteriorly, and the tympanic plate is placed obliquely, not 

 opposite the lateral extremity of the tympanic cavity, but ven- 

 trally to this (cf. Fig. 168). 



Even on the sixth day the position of the head of the stapes 

 may be recognized by the density of the mesenchyme internal to 

 the bottom of the meatus. During the seventh and eighth days 

 the stapes becomes sharply differentiated, and the internal face 

 of the tympanum is established in proportion as the tympanic 

 cavity expands around the cartilage (cf. Fig. 168). Thus the 

 tympanum is faced by ectoderm externally, by entoderm inter- 

 nally, and includes an intermediate mass of mesenchyme, which 

 differentiates by degrees into the proper tympanic substances. 



