August 9, 1901.] 



SCIENCE. 



197 



cartilage bone is always preformed in the 

 peculiar substance known as cartilage, and 

 in this only later is the chondrin matrix re- 

 placed by salts of lime. Membrane bones, 

 on the other hand, never have a cartilage 

 stage. They arise by the direct ossification 

 of connective tissue membranes. Further, 

 investigations seem to show that there may 

 be at least two types of membrane bone, 

 one of which, exemplified by most of the 

 membrane bones of the skull and by the 

 bony plates of the alligator, has arisen by 

 the fusion of the bases of dermal scales or 

 teeth, and the sinking of these to a deeper 

 position. The other type, familiar in the 

 kneepan and the bony strands so well 

 known in the drumstick of the turkey, 

 arises from the ossification of tendons. 



In the sharks the skeleton of the jaws 

 arises from a continuous stroma or anlage 

 on either side of the head. Each of these 

 strands — known as the mandibular arch — 

 becomes interrupted in the middle as it be- 

 comes converted into cartilage. The upper 

 half of each arch forms the upper jaw — the 

 ptery go- quadrate of anatomy, while the 

 lower half in a similar way gives rise to 

 Meckel's cartilage, the skeleton of the lower 

 jaw. These jaws do not articulate di- 

 rectly with the cranium, but the pterygo- 

 quadrate is suspended in front by liga- 

 ments, while behind, besides a ligament, 

 the upper half of the next or hyoid arch 

 intervenes as a hyomandibular element be- 

 tween the jaws and the cranial wall, thus 

 forming a suspensorium for these parts. 



In the teleosts, or bony fishes, where 

 bone largely replaces cartilage in the adult, 

 the hyomandibular still acts as a suspensor, 

 while the ptery go-quadrate, relieved of its 

 functions as the upper jaw, ossifies in two 

 portions ; in front, as a pterygoid bone ; be- 

 hind as a smaller element, the quadrate. 

 The quadrate articulates, on the one hand, 

 with the hyomandibular ; on the other, it 

 supports the Meckelian, the hinge of the 



lower jaw being formed by the articulation 

 of Meckel's cartilage with the quadrate. 



In the vertebrates higher than the fishes 

 the hyomandibular disappears as a sus- 

 pensor, and it is not to be recognized with 

 absolute certainty in this region. As will 

 be seen shortly, there is some evidence that 

 it is not entirely lost, but persists with 

 changed functions. In contrast to what 

 obtains in the fishes, in amphibia, reptiles 

 and birds, the quadrate articulates directly 

 with the cranium in the region of the ear, 

 and forms a suspensor for the lower jaw. 

 In its history in all these groups the quad- 

 rate is preformed in cartilage, and hence, 

 when it ossifies, it becomes converted into 

 cartilage bone. In amphibia, reptiles and 

 birds in the embryonic stages, the Mecke- 

 lian cartilage, of course, articulates with 

 the quadrate, but when the definitive lower 

 jaw is formed, some features are introduced 

 which must be described. In all the bony 

 vertebrates the Meckelian does not furnish 

 the bones of the lower jaw, but these arise 

 as membrane bones arranged round the car- 

 tilage bar. In the amphibia and reptiles 

 the most constant of these bones is a very 

 large, tooth-bearing dentary in front, ex- 

 tending backwards on the outer side of the 

 Meckelian. Further back, on the inner 

 side, is a smaller bone, the splenial, which 

 also may bear teeth. The third of these is 

 the angular, which is placed behind those 

 already mentioned on the lower and inner 

 sides of the proximal end of Meckel's car- 

 tilage. The Meckelian ossifies only at its 

 posterior end, where it articulates with the 

 quadrate, giving rise at this point to a car- 

 tilage bone, the articular. In short, the 

 lower jaw consists of a single cartilage 

 bone, the articular, and three or more mem- 

 brane bones. 



The articular surface of quadrate and ar- 

 ticular presents features which must be 

 mentioned. In all the non-mammalian 

 groups the quadrate has a rounded or some- 



