418 CHARLES CLIFFORD MACKLIN 



The lacrimal (figs. 4 and 17) is a slender spicule of membrane 

 bone lying along the upper and lateral part of the cartilaginous 

 posterior maxillary process, its long axis being directed from 

 behind forward, inward and slightly upward. Its middle part 

 presents a slight thickening, and the naso-lacrimal duct lies im- 

 mediately lateral to it. 



The nasal bone (figs. 1 and 4) is represented in the model as a 

 thin plate, rounded in outline, flattened, and about 1 cm. in 

 diameter, lying upon the part of the upper surface of the tectum 

 nasi which represents the medial surface of the supraconchal 

 prominence. Behind this plate, and lying close to it, are several 

 minute nodules of membrane bone, which also contribute to the 

 formation of the adult bone. The nasal anlagen lies in a mass 

 of dense mesenchyme, which overlies the entire nasal capsule, 

 similarly to the formation described by Fawcett ('10 a and '10 b) 

 in a 30 mm. human embryo. 



The tympanic (figs. 2 and 4) is a short, flat plate of membrane 

 bone, lying beneath the posterior extremity of Meckel's cartilage, 

 and in front of the malleus. Its anterior part is slightly wider 

 than the posterior, and its long axis lies parallel with that of the 

 cartilage, its flattened surface being applied to the latter. The 

 bone is found in an isolated anlage of condensed osteogenic 

 mesench3mie, and is placed lateral to and below the goniale. 

 When compared with the Hertwig model it is seen that the only 

 part which is as yet laid down is the anterior widened extremity, 

 there being no evidence of the ring-like foran of the later bone. 



By the subsequent outward swinging of the ossicles and 

 attached Meckelian cartilages consequent upon the expansion 

 of the region of the middle ear the plane bordering upon Meckel's 

 and Reichert's cartilages is changed in direction, so that its outer 

 surface looks almost directly downward, as shown in the model 

 of Hertwig, instead of outward, as is the case in my model, and 

 at the same time the interval between these two cartilages is 

 relatively narrowed. This interval the tympanic bone comes to 

 occupy, its dorsal half, as yet undeveloped in my embryo, being 

 applied to the cartilage of Reichert in the Hertwig model, the 

 long process of the malleus occupying a position within the ring. 



