DEVELOPMENT OF TUB EAR. 269 



no external and middle ear ; in these animals there is only 

 a labyrinth, an internal ear, situated within the skull. The 

 tympanic membrane, its cavity, and all the connected parts 

 are unrepresented. The middle ear first develops in the 

 Amphibian class, in which a tympanic membrane, a tym- 

 panic cavity, and an Eustachian tube are first found. All 

 these essential parts of the middle ear develop from the first 

 gill-opening, with its surrounding parts, which in the Pri- 

 mitive Fishes (Selackii} remains through life as an open 

 blow-hole, situated between the first and second gill-arches. 

 In the embryos of higher Vertebrates it closes in the centre, 

 the point of concrescence forming the tympanic membrane. 

 The remaining outer part of the first gill-opening is the 

 rudiment of the outer ear-canal. From the inner part 

 originates the tympanic cavity, and further inward, the 

 Eustachian tube. In connection with these, the three bone- 

 lets of the ear develop from the first two gill-arches; the 

 hammer and anvil from the first, and the stirrup from 

 the upper end of the second gill-arch. 174 



Finally, as regards the external ear, the ear-shell (concha 

 aims), and the outer ear-canal, leading from the shell to the 

 tympanic membrane these parts develop in the simplest 

 way from the skin-covering which borders the outer orifice 

 of the first gill-opening. At this point the ear-shell rises in 

 the form of a circular fold of skin, in which cartilage and 

 muscles afterwards form (Fig. 238, p. 247). This organ is 

 also limited to Mammals. Among them, it is originally 

 wanting only in the lowest division, in the Beaked Animals, 

 (Monotrema}. In the others, on the contrary, it appears 

 in very different stages of development and partly also of 

 atrophy. The ear-shell has atrophied in most aquatic 



