ORGANS OF SPECIAL SENSE 297 



as a precursor to chondrification. The distinction between the 

 perilymphatic and cartilaginous zones is most distinct (on the 

 sixth day) on the median surface of the ductus cochlearis and 

 lagena. The differentiation proceeds rapidly, however, and on 

 the eighth day the entire membranous labyrinth is surrounded 

 by a mass of embryonic cartilage, the foundation of the bony 

 labyrinth, excepting around the endolymphatic duct (Fig. 168). 

 Between the bony and membranous labyrinths is a thick layer 

 of perilymphatic tissue composed of very loose-meshed mesen- 

 chyme, which in the course of the subsequent development 

 breaks down to form the perilymphatic space. Portions of the 

 perilymphatic tissue, however, remain attached to the mem- 

 branous labyrinth and form a support for its blood-vessels and 

 nerves. 



The Development of the Tubo-tympanic Cavity, External 

 Auditory Meatus and Tympanum. These structures develop 

 directly or indirectly from the first or hyomandibular visceral 

 cleft and the adjacent wall of the pharynx. In a preceding 

 chapter the early development of this cleft was described; we 

 saw that the pharyngeal pouch forms two connections with the 

 ectoderm, a dorsal one corresponding to a pit-like depression of 

 the ectoderm, and a ventral one corresponding to an ectodermal 

 furrow. The latter connection is soon lost, the ectodermal fur- 

 row slowly disappears, and the ventral portion of the pouch 

 flattens out. In the dorsal connection, however, an opening is 

 formed which closes on the fourth day, and the dorsal division 

 of the pouch then frees itself from the ectoderm and expands 

 dorsally and posteriorly until it lies between the otocyst and the 

 ectoderm, still preserving its connection with the pharynx (Fig. 

 102). 



(a) The Tubo-tympanic Space. The dorsal portion of the 

 first visceral pouch forms the lateral part of the tubo-tympanic 

 space, but the greater portion of the latter is derived from the 

 lateral wall of the pharynx itself, immediately adjacent to the 

 entrance into the first visceral pouch; the region concerned 

 extends from near the anterior edge of the second visceral pouch 

 forwards, and ends a short distance in front of the first pouch. 

 The original transverse diameter of the pharynx in this region 

 increases in the course of development, and a frontal partition 

 grows across the pharynx forming a dorsal median chamber into 



