152 OUTLINES OF CHORDATE DEVELOPMENT 



hind-brain, on the dorsal side of which its tip forms an expan- 

 sion known as the endohjm pluitic sac. These sacs enlarge very 

 considerably, fuse together, and finally, after metamorphosis, 

 form a vascular structure, covering a large portion of the roof 

 and sides of the myelencephalon : they remain throughout 

 life, connecting with the saccule by the narrow endolymph- 

 atic ducts which pass through openings in the walls of the 

 auditory capsules (Fig. 51, F). 



The epithelial lining of the membranous labyrinth becomes 

 truly sensory only in certain patches with which fibers of the 

 VIII cranial nerve are related. These patches are located 

 in the cochlea (3), utricle (1), saccule (1), and the ampullae 

 (3). The perilymph and the cartilaginous and bony labyrinths 

 are laid down around the membranous labyrinth by the sur- 

 rounding mesenchyme. 



The parts of the middle ear develop relatively late and are 

 not well differentiated until after metamorphosis. As in 

 other vertebrates the tubo-tympanic cavity is a derivative of 

 the _pharynx, more precisely of the region of the first gill pouch 

 (the spiracular pouch, between the hyoid and mandibular 

 arches; see below). In the frog this gill pouch is vestigial: 

 it is never perforated, indeed does not contain a cavity, and is 

 represented only by a fold of the endodermal wall of the 

 pharynx which does not quite reach to the surface ectoderm. 

 From the dorsal end of this vestige a rod of cells grows out and 

 terminates as a solid knob, beneath the eye (Fig. 57). During 

 metamorphosis spaces appear in this rod and in the terminal 

 knob, and it moves back into the region of the ear, losing its 

 connection with the pharynx. After metamorphosis it ac- 

 quires a secondary connection with the pharynx, and its distal 

 portion enlarges very considerably, occupying the space be- 

 tween the ear and the integument of the side of the head: 

 the integument here later becomes the tympanic membrane, 

 the expanded cavity is the tympanic cavity, and the narrow 

 connection with the pharynx is the Eustachian tube. 



In the adult the auditory capsule and the tympanic mem- 

 brane are connected by a rod, the columella, extending across 



