342 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



as the exoccipitals, the alisphenoids and the orbitosphenoids. In 



the occipital region the cartilaginous roof gives way to a replacement 

 bone— the supraoccipital — but in the corresponding region further 

 forwards the cartilage either does not develop or if it does develop 

 never becomes replaced by bone. 1 



In addition to the three groups of replacement bones mentioned, 

 the cartilage of the sensory capsules gives way to replacement bones. 

 In the wall of the auditory capsule there develops anteriorly a 

 prootic and in the remaining part of the capsule wall there develop 

 other ossifications differing in number in different groups — as many 

 as four in the case of Teleostean fishes — epiotic, opisthotic, 

 sphenotic and pterotic. The wall of the olfactory capsule similarly 

 becomes replaced by ethmoid bones — varying much in the different 

 groups. The wall of the eyeball in a few cases develops a ring of 

 flattened replacement bones in the substance of the sclerotic. 



In addition to the replacement bones already indicated there 

 develop others in connexion with the visceral arches. Thus we find 

 the upper portion of the mandibular arch becoming replaced by the 

 quadrate, the upper portion of the lower jaw by the articular and the 

 palato-pterygoid outgrowth by palatine and pterygoid elements 

 which however exhibit great differences in their mode of development. 

 The segments of the hyoid and branchial arches become replaced by 

 various hyal and branchial bones in the bony fishes. 



The bony skull is completed by numerous more superficially 

 placed bones — some belonging to the ordinary investment type, 

 others retaining their dental character. The cranial roof immediately 

 in front of the occipital region is typically covered by a pair of 

 parietal bones ; in front of these are a pair of frontals. In the 

 Amniota particularly are developed such additional elements as 

 squamosal, postfrontal, jugal. The Ethmoidal region develops 

 such bones as nasal, prefrontal, lachrymal, septomaxillary. 



In the region of the buccal cavity we find a particularly rich 

 development of investment and dental bones. The function of upper 

 jaw, originally exercised by the palato-pterygoid bar, becomes 

 taken over by new bones — the maxilla and premaxilla — lying 

 external to it, the palato-pterygoid bar becoming shunted inwards 

 except its hinder quadrate portion and no longer forming the margin 

 of the buccal cavity. Behind the maxilla or jugal there may develop 

 a quadrato-jugal : in the region of the palato-pterygoid parts of the 

 palatine and pterygoid are of this origin and' so also are the vomer 

 and parasphenoid. 



Just as the primitive upper jaw becomes replaced function- 

 ally by bones of superficial origin, so also with the lower jaw — 

 the original Meckel's cartilage becoming ensheatbed by splenial and 

 dentary (the latter taking on the tooth -bearing function) with 

 such other bones as angular, supra - angular, and coronoid. In 



1 The pleuroccipital bone of the Dipnoi arises as a close investment of the occipital 

 arch (Agar, 1906). 



