126 



Bulletin American Museum of Natural History. \\o\. XXVII, 



embryo the meatus is filled with cuticle of ectodermal origin and corresponds 

 to the outer portion of the gill tube; the Eustacian canal opens into the 

 pharynx and like it is lined with endoderm. The external meatus and the 

 Eustachian canal are separated by a membrane, the supposed homologue 

 of the septum between the inner and outer gill cavities in fishes ; this accord- 

 ing to Minot (1892, p. 738) is the "Anlage" of the tympanic membrane {cf. 

 Fig. 3, A). The tympanic cavity in man (Minot, /. c, p. 739) arises as a 



^vest^ — 



Jn.ov. 



,^St' 



„/«(p«)-- 



,Mat{Art) 



-mb. ty 



cav.ty — ^ 



St'iex col.)- 

 nuae- 



..tueus. 



Den p 



Mck ■" 



m^mb ty 



Fig. 3. Morphology of the ossicula auditus and adjacent parts; illustrating the quadrate 

 = incus, malleus = articular theory. 



A. Cross section of the auditory region in a human embryo of three months. After Minot. 



B. Hypothetical cross section of the auditory region in an adult pro-mammal. 



A shows the developing ossicula {St'., Mai., In.) wholly outside of the incipient tympanic 

 cavity {cav. ty). 



B sliows the reduced quadrate {Qu = Inc.) and articulare {Art. = Mai) abutting against 

 the columelliform stapes {St'). The tympanic cavity {cav. ty.) is growing up around the ossi- 

 cula. The distal portion of the stapes (Si'. = e.xtra-columella) is supposed on this theory to have 

 given rise to the manubrium mallei. 



P. an., pinna auris. 



m.a.e., external auditory meatus, continuous 



with tu. eus, Eustachian tube. 

 can. sew., semicircular canals. 

 vest., vestibule. 

 fn. ov., fenestra ovalis. 



St'., proximal portion of stapes (columella). 

 In {Qu.), incus (= quadrate). 



Mai. {Art.), malleus (= articulare). 



mb. ty., tympanic membrane. 



cav. ty., tympanic cavity. 



St"., distal portion of stapes (= extra-colum- 



ella). 

 each., cochlea auris. 

 Mck., Meckelian cartilage. 

 Den., dentary. 



dilatation from the tubo-tympanal canal, which grows up from below and 

 surrounds the auditory ossicles. Accordingly these finally appear to be 

 within the tympanic cavity "but," says Minot (/. c, p. 240), "they are, of 

 course covered by the tympanal epithelium or entoderm, and are, therefore 

 morphologically outside the cavity just as the intestine is outside the perito- 

 neal cavity." A similar process takes place in other mammals (cf., Parker, 

 1886, plates showing cross-sections, van Kampen, 1905, pp. 324-329). In 

 the developing Sphe?iodon (Howes and Swinnerton, 1901, pi. v) and lizard 



