XII.] THE AUDITORY OSSICLES. 405 



gracilis of the malleus is the primitive continuation into 

 Meckel's cartilage. 



The malleus and incus are at first embedded in the 

 connective tissue adjoining the tympanic cavity, which 

 ■with the Eustachian tube is the persistent remains of 

 the hyomandibular cleft ; and externally to them a bone 

 known as the tympanic bone becomes developed so that 

 they become placed between the tympanic bone and the 

 periotic capsule. In late fcetal life they become trans- 

 ported completely within the tympanic cavity, though 

 covered by a reflection of the tympanic mucous mem- 

 brane. 



The dorsal end of the part of the hyoid separated 

 from the incus becomes ossified as the tympano-hyal, 

 and is anchylosed with the adjacent parts of the periotic 

 capsule. The middle part of the bar just outside the 

 skull forms the stylo-hyal (styloid process in man) which 

 is attached by ligament to the anterior comu of the 

 hyoid (cerato-hyal). The tympanic membrane and ex- 

 ternal auditory meatus develop as in the chick (p. 166). 



The ribs and sternum appear to develop in Mammals as in 

 Birds (p. 234). 



The pectoral girdle, as in Birds (p. 234), arises as a con- 

 tinuous plate of cartilage, the coracoid element of which is how- 

 ever much reduced. 



The clavicle in Man is provided with a central axis of car- 

 tilage, and its mode of ossification is intermediate between that of 

 a true cartilage bone and a membrane bone. 



The pelvic girdle is formed in cartilage as in Birds, but in Man 

 at any rate the pubic part of the cartilage is formed independently 

 of the remainder. There are the usual three centres of ossification, 

 which unite eventually into a single bone — the innominate bone. 

 The pubis and ischium of each side unite ventrally, so as com- 

 pletely to enclose the obturator foramen. 



