192 



EMBRYOLOGICAL TYPES 



perforated, converting the shelf into a half- ring. In this way 

 the semicircular canals are formed. The cavity of the auditory 

 sac contains endolymph. Between the wall of the sac and 

 the capsule of connective tissue which surrounds it, is the 

 perilymph. The capsule eventually becomes cartilaginous, 

 and later on, bony ; but certain apertures are left. One of 



i p. / 



vs. 



Fig. 86. — Transverse section through an embryo of Rana showing the 

 formation of the ears. 



an, auditory nerve ; av, auditory vesicle ; bv, blood-vessels running in 

 the visceral arches ; eg, external gills ; g, gut ; h, heart ; hb, hindbrain ; n, 

 notochord ; p, pericardium ; ta, truncus arteriosus ; vs, ventral sucker. 



these is the fenestra rotunda, and another is the fenestra ovalis 

 on to which the base of the columella auris fits. The outer 

 end of the columella auris is applied to the thin lateral wall of 

 the tympanic cavity which forms the tympanic membrane. 



It may be mentioned here that, remarkable as it may seem, 

 the ears are responsible for the formation of the so-called calci- 

 gerous glands, or glands of Schwammerdamm. These glands 



