324 OUTLINES OF CHORDATE DEVELOPMENT 



The mesenchyme surrounding the otocyst differentiates into 

 various structures during the development of the otocyst. 

 First is formed a membranous layer which fuses with the ex- 

 ternal surface of the complex otocyst and its derivatives, 

 forming the membranous labyrinth. Surrounding this the 

 mesenchyme forms a loose tissue which becomes the peril ymph, 

 and around all of this comes finally a dense mesenchyme where 

 the cartilaginous and later the bony labyrinths are laid down. 



We have thus far described only the structures of the internal 

 ear. It remains now to mention the chief facts regarding the 

 development of the middle and outer ears. These develop 

 partly from the pharyngeal cavity, and partly from the region 

 of the hyomandibular visceral pouch and the surrounding 

 mesenchyme. The hyomandibular pouch develops in two 

 parts, a large ventral portion corresponding in general with 

 the typical gill-pouch, and a smaller dorsal portion (Fig. 130, 

 B}. The former is transitory and disappears without becoming- 

 perforated, while the latter, which is perforated for a short 

 time only, enlarges and becomes differentiated as a part of the 

 middle ear or tympanic cavity. The major portion of this 

 cavity is, however, derived from the cavity of the pharynx 

 adjoining the dorsal portion of the hyomandibular pouch. 

 This part of the pharynx becomes incompletely cut off from 

 the remainder of the pharyngeal cavity by a horizontal shelf or 

 partition, a narrow slit remaining open in the mid-line. The 

 narrow pharyngeal space thus formed, and the dorsal portion 

 of the hyomandibular pouch, enlarge distally, between the 

 otocyst and the surface of the head, as the rudiment of the 

 tympanic cavity; the narrow medial portion of the pharyngeal 

 space becomes the Eustachian tubes, opening into the pharynx 

 by the slit mentioned. 



The mesenchyme of the dorsal wall of the tympanic cavity 

 becomes differentiated into the two inner auditory ossicles. 

 The cavity then extends dorsally around either border of this 

 mesenchymal region, which is thus formed into a stalk contain- 

 ing the two ossicles and connecting the surface of the head 

 with the wall of the otocyst (twelve days). 



