220 OUTLINES OF CHORDATE DEVELOPMENT 



with the teeth (see below), while in the lower jaw Meckel's 

 cartilage becomes surrounded by the dentary and angular; the 

 dentary then connects with the infrarostrals of Meckel's carti- 

 lage. Paired vomers beneath the olfactory capsules, and pala- 

 tines across the anterior margins of the orbits develop next. 

 The pterygoids develop along the inner faces of the palato- 

 quactrate cartilages, and the squamosals along their outer sur- 

 faces, finally extending back over the auditory capsules. The 

 quadrato-jugal represents a dermal element developed in con- 

 nection with the posterior angle of the palato-quadrate carti- 

 lage, and fused with the cartilage quadrate bone of the palato- 

 quadrate itself. With the exception of this element the derm 

 bones remain easily separable from the cartilage bones and the 

 remains of the chondrocranium, even in the skull of the fully 

 grown frog. 



3. The Teeth 



Teeth are present in the adult on the premaxillae, maxillae, 

 and vomers. During the larval period the teeth are function- 

 ally replaced by the horny "jaws" and " teeth," so that the 

 true teeth develop relatively late. They form independently 

 of the bones with which they are later associated. During 

 metamorphosis a series of dermal papillae forms around the 

 margin of the upper jaw and covering the vomers. These are 

 the tooth germs; they project into an associated thickening of 

 the epidermis. The Malpighian cells of the epidermis covering 

 each tooth germ become the enamel organ and secrete a layer 

 of enamel over the surface of the hollow cone of the dentine 

 which is formed by the surface of the dermal papilla. The 

 cellular core of the papilla remains as the pulp cavity of the 

 tooth. The teeth are elongated by the formation of bony 

 tissue at their bases, and they gradually push through the 

 epidermis. This basal mass of bony tissue serves also to attach 

 the teeth to the inner sides of the jaw, and to the surface of the 

 posterior part of the vomer; their attachment does not occur 

 until some time after metamorphosis. 



