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ME. W. K. PAEKER ON THE STEUCTUEE AND 



(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. 



(d) A pair of cartilaginous nasal capsules. 



Further, that in the Mammalia, as in the other Vertebrata the development of the 

 skull of Avhich 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 liird the hyoidean arch remains distinct from the mandibular; Avhilst 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 cartilaginous part 



