SOCIETY OP THE UNIVERSITY OF ABERDEEN. 67 



well seen. As the cartilage passes forwards its anterior part bends 

 sharply round towards the middle line this bend takes place at 

 a point opposite the interval between the germs of the lateral incisor 

 and canine teeth. The cartilage is enlarged and club-shaped at its 

 anterior end, but becomes rapidly less when traced back towards the 

 mental foramen, while behind this point its diameter increases some- 

 what (Fig. 6). The anterior ends of Meckel's cartilages, while they 

 approach each other closely, are distinctly separated by a layer of 

 mesodermic cells. There is no indication of commencing ossification 

 in the cartilage. The membrane bone of the jaw lies on the outer 

 side of the cartilage, and, following the bend described by it at its 

 anterior end, comes to lie in front of the cartilage, so that coronal 

 sections here (Fig. 6) show only the membrane bone. In front the 

 membrane bone approaches the bone of the opposite half, almost 

 meeting in the middle line. Tracing the membrane bone from behind 

 forwards, it first of all forms a simple lamella in which the coronoid 

 process and angle are already distinctly outlined. Still more forwards 

 in the region of the last milk molar tooth germ a distinct tooth 

 gutter begins to be formed, a coronal section of jaw a little in front of 

 this having a Y-shaped appearance, with lateral and mesial walls. 

 A lateral and mesial wall to the tooth gutter can be traced as far 

 forward as the canine tooth, where the mesial wall disappears, the 

 lateral only being continued forward. The mental foramen is repre- 

 sented as a much-elongated slit in the lateral wall, with the mental 

 branch of the inferior dental nerve passing out in close relation with 

 the posterior border of the foramen. 



In an <'inl>ri/<> 31 mm. in leiujth, coronal sections of lower jaw 

 present much the same appearances as in the embryo of 28 mm. The 

 anterior extremities of Meckel's cartilage are more flattened from 

 before back, and where they touch the membrane bone, just opposite 

 the interval between the lateral incisor tooth germ and that of the 

 canine, the cartilage cells are becoming somewhat enlarged prepara- 

 tory to ossification. 



In ait I'lithryo 36 mm. in length the anterior extremity of Meckel's 



