406 



On the Structure of the Skull in the Pig. [June 19, 



Further, that in the Mammal, as in the other Yertebrata the development 

 of the skull of which has been examined, the basicranial plate grows up as 

 an arch over the occipital region of the skull, and coalesces with the 

 auditory capsules, laterally, to give rise to the primordial skeleton of the 

 occipital, periotic, and basisphenoidal regions of the skull. The trabecule 

 become fused together, and, imiting with the olfactory capsules, give rise 

 to the presphenoidal and ethmoidal parts of the cranium ; and the 

 moieties of the skull thus resulting from the metamorphosis of totally 

 different morphological elements become united and give rise to the 

 primordial cranium. 



As in the Salmon and Fowl, the second pair of praeoral arches give 

 rise to the pterygo-palatine apparatus ; in the Frog this arch is late in 

 appearance, and is never distinct from the trabecular and mandibular bars, 

 serving as a conjugational band between them. The mandibular arch, 

 which in the Salmon becomes converted into Meckel's cartilage, the 

 os articulare, the os quadratmn, and the os metapterygoidemn, in the Frog 

 into Meckel's cartilage and the quadrate cartilage (which early becomes 

 confluent with the periotic capsule), in the Bird into Meckel's cartilage, 

 the os articulare, and the os quadratum (which articulates movably with 

 the periotic capsule), in the Pig is metamorphosed into Meckel's cartilage 

 and the malleus, which is loosely connected with the tegmen tympani, 

 an outgrowth of the periotic capsule. 



Meckel's cartilage persists in the Fish and in the Amphibia, but disap- 

 pears early in the Bird, and still earlier in the Mammal. The permanent 

 ossifications of the mandible are all membrane-bones in Fish, Frog, and 

 Fowl, but in the Mammal (exceptionally) the ramus has a cartilaginous 

 foundation. The hyoidean becomes closely united with the mandibular 

 arch, and then segmented, in the Fish, into the hyo-mandibular, the 

 stylo-hyal, cerato-hyal, and hypohyal — the hyo-mandibular, or proximal 

 segment, articulating with the outer wall of the periotic, and many 

 of the segments of the arch becoming dislocated. 



In the Frog, the hyoid also becomes segmented, but only after ex- 

 tensive coalescence with the mandibular arch. The proximal segment 

 becomes the suprastapedial (hyo-mandibular) with its extrastapedial 

 process, and, extending inwards as mediostapedial and interstapedial, 

 articulates with the stapes, developed by segmentation from the outer 

 wall of the auditory capsule. The stylo-hyal is dislocated and becomes 

 connected with the auditory capsule below the stapes (opisthotic region). 



In the Bird, the hyoidean arch remains distinct from the mandibular. 

 Whilst in its primordial condition it coalesces by its incurved apex 

 with the auditory capsule in front of the promontory, before the stapedial 

 plug is segmented. It then chondrifies as three distinct cartilages — an 

 incudal, a stylo-hyal, and, distally, a cerato-hyal. The stapes becomes free 

 from the auditory capsule, but remains united with the cartilaginous part 

 of the incus (mediostapedial) ; the ascending part is largely fibrous 



