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.) 
(b) The Haternal 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 shg¢ht 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 (ef. 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. 
