ORGANS OF SPECIAL SENSE 299 
lary portion of the colliculus palato-pharyngeus corresponds to 
the palatine processes of mammals; the mandibular portion is 
peculiar to Sauropsida. 
If the interior of the pharynx and oral cavity of a ten-day 
chick be examined (Fig. 174 B), it will be found that the col- 
liculus has undergone important changes. Its maxillary or an- 
terior division divides in two limbs, crura superior and inferior, 
diverging anteriorly and separated by a depression which con- 
tinues the nasal cavity backward; its free posterior end extends 
farther backwards than before, and is more elevated. The 
bounding sulci are both deeper than before. The sulcus tubo- 
tympanicus, with which we are specially concerned, now extends 
on to the median surface of the hyoid arch. Subsequently, the 
crura superiores of the opposite side meet in the middle line and 
fuse together; in a similar fashion the posterior ends of the col- 
liculi fuse; thus the sulci tubo-tympanici open into a dorsal 
chamber common to both, which communicates with the ventral 
division of the pharynx by a slit remaining between the two 
fused areas. The crura inferiores also approach one another 
in the middle line but do not fuse, thus leaving the typical split 
palate of birds in front of the fused lower ends of the crura super- 
iores. In this way the typical adult condition of the bird’s 
palate is established. 
From this description it will be seen that only the most lateral 
portion of the tubo-tympanic cavity is directly derived from 
the first visceral pouch. In later stages it is quite impossible 
to say exactly what part, but it is quite certain that it lies within 
the tympanic part of the cavity. About the end of the fifth 
or the beginning of the sixth day the tubo-tympanic canal begins 
to enlarge distally to form the tympanic cavity proper (ef. Fig. 
168); the auditory ossicles (see chapter on skull) are beginning 
to form just above its dorsal extremity, and as the tympanic 
‘avity enlarges it expands around them, displacing the mesen- 
chyme, and finally meets above the auditory ossicles, so that 
these appear to he within it, though as a matter of fact the rela- 
tion is analogous to that of the entodermal alimentary tube to 
the body-cavity. The process of inclusion of the auditory ossicles 
is not, however, concluded until about the twelfth day. The 
blind end of the tympanic cavity attains a level dorsal to the 
external auditory meatus. (See below.) 
