THE SENSE ORGANS 



605 



In the higher vertebrates the cartilaginous otic capsule is subsequently 

 converted into bone by the process of endochondral bone formation. 

 But in the axis of the spiral cochlea the connective tissue is converted 

 directly into bone, after the manner of membrane bone formation. The 

 contrast between the modes of formation of the inner and outer portions 

 of the bony part of the cochlea seems to have no phylogenetic significance. 



HYOMANDIBULAR 



-ARTICULAR--^^ 



DENTAR 



MANDIBULAR 

 A. ELASMOBRANCH 



HYOMANDIBULAR CCOLUMELLA) 



QUADRATE 



ARTICULAR w_^ 

 DENTARX^ 

 MANDIBULAR 



2ND 

 MANDIBULAR 



B. TELEOST. 



STYLOID PROCESS 



(HYOID^ r 



STAPES CHYOMANDIBULARJ 



INCUS CQUADRATEJ— 



MALLEUS CARTICULAR) 



MECKEL'S 



CABTILAGE CMANDIBULAR^^ 



HYOID 



DENTARY- 

 STYLOHYOID LIGAMENT CHYOID) - 



HYOID BONE 



C. AMPHIBIAN AND REPTILE. D. MAMMAL. 



Fig. 501. — Diagrams of the first and second visceral arches in A, Elasmobranch, 

 B, Teleost, C, Amphibian and Reptile, and D, Mammal, illustrating the transformation 

 of the hinge of the jaw of lower vertebrates into the malleus and incus of the mammal. 

 The third earbone, the stapes, comes from the hyomandibular. (Redrawn after Gegen- 

 baur and Stempell.) 



The middle ear and the Eustachian tube which leads from it to the 

 pharynx are both derived directly from the spiracular pouch, a diverticu- 

 lum of the pharynx. Consequently, both, like the pharynx from which 

 they evaginate, are lined by endoderm. To meet this endodermic 

 diverticulum, the ectoderm invaginates to form the external auditory 

 meatus. The two-layered membrane between the two invaginations, 



