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 les 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 
