CHAP. X.] TEE DEVELOFMENT OF THE CAT. 339 



the adult really consists of two layers of bone which have anchy- 

 losed together. 



Besides these four elements, three other distinct ossifications arise 

 in the primitive cartilaginous auditory capsule, spread and coalesce 

 to form the petrous and mastoid portions of the temporal bone. 

 They are distinguished by their various relations to diflerent parts 

 of the bony and membranous labyrinths. The first of these is called 

 the pro-otic, and forms the upper rim of the fenestra oralis, and 

 invests the anterior vertical semicircular canal. It ultimately forms 

 that jDart of the petrous bone visible inside the skull and part of the 

 mastoid. The second ossification is the beginning of the opisthotic. 

 It forms the lower rim of the fenestra ovalis, and entirely surrounds 

 the fenestra rotunda. It constitutes the lower part of the petrous 

 bone. The third ossification results in a bono called the epiotic, 

 which invests the posterior vertical semicircular canal. It ultimately 

 forms the mastoid process. 



The auditor^/ ossicles are formed, in part at least, by ossifications 

 of the proximal ends of the cartilages of the first and second visceral 

 arches. 



That of the first arch, or JSIcckcl's cartilage, long persists on the 

 inner side of the mandibular ramus, but its upper end ossifies as the 

 processus gracilis and body of the maUcus. The proximal cud of 

 the second arch, which appears at one period to be bifurcated, has 

 as ossifications of one of its upper ends, the tympano and stylo-hyals, 

 while the other end becomes the body and long crus of the incus, 

 the latter being its apex and summit. 



The stapes is of a difibrent nature, being a small part of the 

 cranial wall which has grown out, become separated, and secondarily 

 connected with the upper part of the second arch. 



The lower end of this arch ossifies as the cpi-, cerato- and basi-hyals. 



The OS orbiculare is the uppermost end of that part, the lower 

 portion of which is the thyro-hyal. 



The thyro-hyals are the solitary ossifications of the third viscera 

 arch. 



Thus, in recapitulation, the skull as a whole may be said to arise 

 as follows : — the cranial walls and floor behind, are formed from the 

 parachordal and auditory capsular cartilages ; in front, by the trabe- 

 cula3, the ethmovomerine plate, alas and orbital cartilages ; above, 

 from membrane. The visceral arches form the jaws and hyoid, in 

 conjunction with downgrowths from the ethmovomerine plate. The 

 trabecula) extend forwards between the floor of the brain cavity and 

 the roof of the pharynx, closely embracing the pituitary fossa, into 

 which the roof of the primitive buccal cavity temporarily (see below, 

 p. 343) projects. 



§ 11. The LiJiBs of the embryo begin as two slight prominences on 

 the surface of each lamina ventralis. They subsequently take the 

 form of somewhat cylindrical processes, each with a flattened terminal 

 expansion (Fig. 152). This expansion becomes divided at its distal 

 end into five processes (the digits), the pre- axial one of which in 



z 2 



