SOCIETY OF THE UNIVERSITY OF ABERDEEN. 



79 



case even in mammals in which in the adult the two halves of the 

 lower jaw remain separate. Again, in the examination of the lower 

 jaw of the turtle I found that there very frequently exists a calci- 

 fication of Meckel's cartilage in the region corresponding to that 

 which in man becomes ossified (Fig. 20). 



At a comparatively late stage 

 in the development of the lower 

 jaw certain airwoi'i/ cartilaginous 

 nuclei appear in connection with 

 the primary membrane bone (Figs. 

 21 and 22). Thus there is a well- 

 defined, wedge-shaped condylar 

 cartilage and a smaller coronoid cartilage. In addition to these there 

 are also smaller cartilaginous nuclei along the margins of both alveolar 

 walls in front as well as along the front of the lower border of the 

 jaw. These cartilaginous nuclei have no separate centres of ossifi- 

 cation but become resorbed and ossification extends into them from 

 the neighbouring membrane bone. In the human lower jaw I have 

 not observed a definite angular cartilaginous nucleus, although in 



in 



/&?- 



Ci. 



Fig. 20. Right half of the lower jaw of turtle 

 viewed from the inner aspect (). Shows 

 calcification (x.) in Meckel's cartilage (mk.). 

 ar., articular; an., angular; co., coronoid; 

 d., dentary. 



C3 



C.J X 



M.C 

 Fig. 21. Fig. 22. 



Scheme of /lie eiereluf>iii human lim-trjaw (right half). 



Fig. 21 viewed from the outer and Fig. 22 from the inner aspect. The figures show the single 

 membrane bone (Hi, titulary) with its accessory cartilaginous nuclei. Cy., condyloid cartilage ; 

 Cr., coronoid cartilage; C. 1., cartilaginous nucleus at outer alveolar margin; C. 2., car- 

 tilaginous nucleus at inner alveolar margin; C. 3., cartilaginous nucleus at lower border; 

 M.C , Meckel's cartilage, with X indicating the part which becomes incorporated with the 

 lower jaw. 



many other mammals a distinct and well-defined angular cartilaginous 

 nucleus exists (Figs. 23 and 24). These various accessory cartila- 

 ginous nuclei do not indicate separate elements, but are an adaptation 

 to the growth of the jaw. 



In conclusion, I wish to express my thanks to Professor Reid for 



