414 CHARLES CLIFFORD MACKLIN 



stage, in the form of enlargement of the cells, while in the same 

 region, in the 36 mm. stage, he mentions ossification of the 

 perichondrium with vacuolization and enlargement of the cells. 

 Again, in the 55 mm. stage, he figures this region of Meckel's 

 cartilage almost surrounded by bone, and undergoing resorption. 

 Hertwig's model shows a complete investment of covering bone 

 in this region. My model is intermediate between the 36 and 

 55 mm. stages of Low, the cartilage being only half surrounded 

 by bone, and ossification just commencing within. 



Of the auditory ossicles all are represented in the model (figs. 

 2 and 3). The malleus, as has been stated, is directly continuous 

 ventrally with the shaft of Meckel's cartilage. Dorsally it is 

 cut off from the incus by perichondrium, though in the model 

 the two are represented as continuous, a lateral furrow marking 

 the intervening boundary. The head of the malleus is large 

 and rounded, and lies just lateral to the tegmen tympani, being 

 separated by connective tissue; its manubrium is long, thick- 

 ened proximally, and its tip is closely applied to the promontory 

 of the cochlea, a condition which is strikingly different from that 

 found in the model of Hertwig, where there is a considerable 

 interval between these parts, caused apparently by the expansion 

 of the middle ear region, which has thrust the upper end of 

 Meckel's cartilage, with its affixed structures, outward. In the 

 angle between the manubrium and Meckel's cartilage the tym- 

 panic and goniale are seen. The goniale, which represents the 

 future anterior process, is at present unattached to the malleus, 

 and remains so till the end of the fifth month (Broman '99) . 



The incus is completely separated from the malleus, in front, 

 and from the otic capsule, behind, by perichondrium, and pre- 

 sents a body to which is attached a long and a short limb, 

 morphologically resembling the adult condition. The incudo- 

 stapedial articulation is formed of condensed mesenchyme, and 

 marks the apex of a right angle, open upward, outward and back- 

 ward, formed by the long limb of the incus and the two limbs of 

 the stapes, and in this angle the facial nerve is to be found. 



The stapes at this stage is circular in form, the base being 

 imperfectly developed. The limbs are represented by well- 

 defined, round cartilaginous rods. 



