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 (cf. Fig. 

 168); the auditory ossicles (see chapter on skull) are beginning 

 to form just above its dorsal extremity, and as the tympanic 

 cavity enlarges it expands around them, displacing the mesen- 

 chyme, and finally meets above the auditory ossicles, so that 

 these appear to lie 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.) 



