196 EDWAKD MCCRADY, JE. 



The principal point to be noticed here is the extraordinary 

 anterior process of the malleus. This process extends in a 

 semicircle halfway around the tympanic annulus, and it per- 

 sists in the adult. It is derived, not from Meckel's cartilage, 

 which is also shown in the figure, but from a membrane bone, 

 presumably the prearticular, which partly encloses the pos- 

 terior end of Meckel's cartilage just as the dentary encloses 

 the anterior end. The portion of Meckel's cartilage lying in 

 the groove of the prearticular degenerates without ossifi- 

 cation. All of the manubrium and possibly also the head and 

 neck of the malleus, however, are formed from the extreme 

 tip of Meckel's cartilage. The foramen shown in the an- 

 terior process admits the passage of the chorda tympani nerve. 



tympanic Me ckel's 



articulations T^^>^r^ annulus 



manubrium "^anterior process 

 . stapes ^w,u^ Incus of malleus of ma/Jeus 



Fig. 62 Eeconstruction of ossicles and tympanic annulus 49 days after birth 

 showing the extraordinary anterior process of the malleus. Dorsal view. 



My tentative view of the homologies of these structures is : 

 The original mandibular arch becomes subdivided into an 

 upper palatoquadrate and a lower Meckel's cartilage. The 

 posterior end of the palatoquadrate ossifies to form the 

 quadrate of reptiles, which becomes the incus of mammals. 

 The posterior end of Meckel's cartilage ossifies into the 

 articulare of the reptiles, which becomes the manubrium, and 

 possibly the head and neck, of the malleus of mammals. The 

 anterior process of the malleus, which is exceptionally large 

 in the opossum, is formed from a membrane bone, the pre- 

 articular of reptiles. Another membrane bone, the angulare 

 of reptiles, becomes the tympanic annulus of mammals. 



This accounts for all but the stapes, the only one of the 

 ossicles about which there has been general agreement in the 



