ME. W. K. PARKER ON THE STRUCTURE AND 
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(a) A cartilaginous basicranial plate embracing the notochord, and stopping, like it, 
behind the pituitary body. 
( b ) Paired cartilaginous arches, of which two are preoral, while the rest are postoral. 
(c) A pair of cartilaginous auditory capsules. 
{cl) A pair of cartilaginous nasal capsules. 
Further, that in the Mammalia, as in the other Vertebrata 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 trabeculae become fused together, and, uniting 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 morpho- 
logical elements become united to give rise to the primordial cranium. 
As in the Salmon and Fowl, the second pair of preoral 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 quadratum, and the os metapterygoideum, 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 meta- 
morphosed into the malleus, which is loosely connected with the tegmen tympani, an 
outgrowth of the periotic capsule. 
Meckel’s cartilage persists in the Fish and the Amphibia, but disappears early in the 
Bird, and still earlier in the Mammal. The permanent ossifications formed outside the 
primary mandible are all membrane-bones in Fish, Frog, and Fowl, but in the Mammal 
(exceptionally) the ramus has a cartilaginous foundation. In the Fish the hyoidean 
arch becomes closely united with the mandibular, and then segmented into the hyo- 
mandibular, the stylohyal, ceratohyal, and hypohyal — the hyomandibular or proximal 
segment articulating with the outer wall of the periotic, and many of the segments 
becoming dislocated. 
In the Frog the hyoid also becomes segmented into three pieces. The middle 
segment becomes the suprastapedial (hyomandibular) with its extrastapedial process, 
and, extending inwards as mediostapedial, articulates with the stapes, developed by 
segmentation from the outer wall of the auditory capsule, the proximal part, or inter- 
stapedial, intervening. The stylohyal 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 stylohyal, and, distally, a ceratohyal. The stapes 
becomes free from the auditory capsule, but remains united with the cartilaginous part 
