DEVELOPMENT OF THE SKULL IN THE PIG-. 



331 



of the incus (mediostapedial) ; the ascending part islargely fibrous (suprastapedial), and 

 the part loosely attached to the mandibular arch is the elongated extrastapedial. The 

 short stylohyal afterwards coalesces with the body of the upper or incudal segment by 

 an aftergrowth of cartilage (the " interhyal " tract) ; a long membranous space inter- 

 venes between it and the glossal piece (ceratohyal). Thus the " columella " of the 

 Bird is formed of three hyoidean and one periotic segment. 



In the Pig the hyoidean arch is distinct, but articulates closely with the mandibular ; 

 its upper segment (hyomandibular) is converted into the incus, and becomes connected 

 with the stapes, its disciform apex being ossified as the " os orbiculare." The stylo- 

 hyal is dislocated and coalesces with the opisthotic region of the auditory capsule. 



The views which have hitherto been entertained respecting the mode of development 

 of the ossicula auditus of the Mammalia fall under four heads :— 



1. According to Reichert*, the malleus and incus both result from the metamorphosis 

 of the cartilaginous skeleton of the mandibular arch, while the stapes proceeds from an 

 after segment of the hyoidean arch, Avhich becomes separated and imbedded in the outer 

 wall of the auditory capsule. 



The latest writer on the subject, SEMMERf, supports Eeichert's views in the main, 

 but is not quite sure about the origin of the stapes. 



2. GiiNTHER J holds that not only the malleus and the incus, but the stapes as well, 

 # are the product of the metamorphosis of the skeleton of the mandibular arch. 



3. Magitot and Eobin§, on the other hand, maintain that the malleus only takes its 

 origin from the skeleton of the mandibular arch. They consider the incus and stapes 

 to arise independently, but do not expressly refer them to the skeleton of the second 

 postoral visceral arch. 



4. Professor Huxley ||, arguing from the anatomy of the mandibular and hyoidean 

 arches in the lower Vertebrata, has put forward the view that the malleus of the Mam- 

 malia is the product of the metamorphosis of the proximal end of the cartilaginous 

 skeleton of the mandibular arch, while the incus proceeds from the proximal end of the 

 hyoidean arch, and is the homologue of the " suprastapedial " of the Sauropsida. He 

 expresses no opinion respecting the origin of the stapes. 



* " Ueber die Yisceralbogen der WirbeltHere," Mullek's Archiv, 1837. 



t TJntersuehimgen iiber die Entwickelung der Meckel'schen Knorpels und seiner Nacbbargebilde. Dorpat, 

 1872. 



t Beobachtungen iiber die Entwickelung des Gebororganes bei Menschen und hiiheren Saugethieren. Leipzig, 

 1842. 



§ " Memoire sur un organe transitoire de la oie foetale designe dans le nom cartilage de Meckel," Annalcs dcs 

 Sciences Naturelles, ser. 4, i., xviii. 1862. 



11 " On the Representatives of the Malleus and the Incus of the Mammalia in the other Vertebrata," Pro- 

 ceedings of the Zoological Society, 1869. 



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