422 CHARLES CLIFFORD MACKLIN 



cranially by a distinct indentation, the representative of the 

 future sigmoid notch, which separates the anlagen of the condylar 

 and coronoid processes. Just external to the latter is the zygo- 

 matic bone, and internal to it is the cartilaginous pars temporalis 

 of the sphenoid. The lateral surface of the plate shows, about 

 the junction of the anterior and middle thirds, a distinct open- 

 ing, the mental foramen, and at this point there is a bend in the 

 bone, the direction of the long axis changing from downward 

 and forward to upward, inward and forward. A short distance 

 above the lower border of the bone, and parallel with it, a groove 

 may be seen upon the lateral surface, more distinct posteriorly 

 than anteriorly. This is also shown in the illustrations of Low's 

 older specimens, and appears to represent the line of insertion of 

 the masseter. 



The anterior half of the bone shows an inner table, regarded 

 by some authors as the splenial element, which separates the 

 vessels and nerves from Meckel's cartilage, becoming continuous 

 anteriorly with the ossified perichondrium immediately sur- 

 rounding the cartilage. About the center of the long axis of the 

 bone the so-called splenial element dwindles in height, and 

 terminates as a thin, backwardly projecting spindle of bone, to 

 which the mylohyoid muscle is attached. Anteriorly the mem- 

 brane bone overlaps the termination of the cartilage of Meckel, 

 and thus shows the condition observed by Low as early as the 

 18 mm. stage. 



On the whole, the shape of the mandible suggests the adult 

 condition, though it is considerably more slender, and the angle 

 is more obtuse. Immediately caudo-medial to the latter is the 

 tip of Reichert's cartilage. There are no accessory cartilaginous 

 nuclei observable, as Low describes in an older model. 



Low ('05) has held that each half of the mandible is laid down 

 as a single skeletal element, the dentary, the so-called splenial 

 element being simply an extension of this. The condition in my 

 embryo supports this finding, the main portion of the bone being 

 fonned of the dentary, the splenial element being but a thin 

 lamella of osseous tissue, directly continuous below with the 



