SOCIETY OF THE UNIVERSITY OF ABERDEEN. 65 



mammalian orders and making a few microscopic sections of different 

 regions of these. To understand lower jaw ossification it is necessary 

 to obtain a complete series of embryos of any one mammal, and to 

 make and mount complete serial sections of the head or lower jaw ol 

 these embryos. Further, it is necessary, especially in the earlier 

 stages where the changes are very rapid, to make sections in different 

 planes, i.e., coronal, sagittal and horizontal. After this manner I have 

 examined the developing lower jaw of man and various mammals, 

 e.g., pig, rabbit, mouse, mole and bat. I have a fairly complete series 

 showing human foetal lower jaw in its different stages, and in the 

 present paper I describe the development of the hunum lower jaw. 



In sections of the mandibular arch of a human embryo 10 mm. 

 in l<'ii< i tli, 1 there is no indication of Meckel's cartilage. 



Fig. 4. Fig. 5. 



Figs. 4 and 5. Horizontal sections of the lower jaw of a human embryo 18 mm. in length ( x 8). 

 Fig. 4 shows the lamella of membrane bone, J., lying to the outer side of the inferior dental 

 nerve, N. Fig. 5 at a lower level shows the relation of the membrane bone, J., to Meckel's 

 cartilage, M.C. 



Ill mi. embryo 15 nun. in length. Meckel's cartilage is already well 

 formed and passes forwards towards the middle line, almost meeting 

 but not fusing with its fellow of the opposite side. 



In (tii etnln-i/o IS nun. in length ossification is present as a very 

 delicate lamella of bone developed in the mesoderm on the outer 

 aspect of Meckel's cartilage (Figs. 4 and 5). This lamella extends 

 forwards almost to the middle line. The inferior dental nerve lies 

 between this lamella and Meckel's cartilage, and its dental branch 

 passes over a notch on the upper border of the lamella. 



In an, embryo 28 mm. in length ossification has extended so that 



1 The crown-rump measurement has been adopted in taking the length of all 

 embryos and foetuses mentioned in the present paper. 



9 



