DEVELOPMENT OF THE SENSE ORGANS 387 



tive tissue in which the auditory ossicles are developed and for a time embedded. 

 The pneumatic cells are formed at the close of fetal life. 



The development of the auditory ossicles has been described by Broman 

 (Verh. Anat. Gesellsch., Kiel, Anat. Anz. Suppl., vol. 14, 1898). According 

 to his account, the condensed mesenchyma of the first and second branchial 

 arches gives rise to the ear ossicles. This tissue is divided in the proximal part 

 of the arches into lateral and median masses. 



The malleus is formed from the distal portion of the median mesenchymal 

 mass of the first arch, along with Meckel's cartilage of the mandible. The 

 cartilaginous anlage of the malleus is continuous with Meckel's cartilage. Be- 

 tween it and the incus is an intermediate disk of tissue, which later forms an 

 articulation. When the malleus begins to ossify it separates from Meckel's 

 cartilage. 



The incus is derived from the proximal portion of the lateral mesenchymal 

 mass of the first branchial arch. The anlage of the incus unites with that of the 

 capsule of the labyrinth and separates from it only when its cartilage develops. 

 It is early connected with the anlage of the stapes, and the connected portion be- 

 comes the crus longum. Between this and the stapes an articulation develops. 



The stapes and Reichert's cartilage are derived from the median mesenchy- 

 mal mass of the second branchial arch. The mesenchymal anlage of the stapes is 

 perforated by the stapedial artery, and its cartilaginous anlage is ring-shaped. 

 This form persists until the middle of the third month when it assumes its adult 

 structure and the stapedial artery disappears. 



Fuchs, after studying the development of the ear ossicles in rabbit embryos, concludes: 

 (1) that the stapes is derived from the capsule of the labyrinth; (2) that the malleus and incus 

 arise independent of the first branchial arch. 



The External Ear. — The external ear is developed from and about the first 

 branchial groove. The auricle arises from six elevations which appear three on 

 the mandibular and three on the hyoid arch (Fig. 368). These anlages were first 

 described by His. 



They are numbered ventro-dorsally on the mandible and in the reverse direction on the 

 hyoid arch. Caudal to the hyoid anlages a fold of the integument is formed, the hyoid helix 

 or auricular fold. A similar fold forms later dorsal to the first branchial groove and unites with 

 the auricular fold, to form with it the free margin of the auricle. The point of fusion of these 

 two folds marks the position of the satyr tubercle, according to Schwalbe. Schwalbe derives the 

 tragus from mandibular hillock 1; the helix from mandibular hillocks 2 and 3; the antihelix 

 from hyoid hillocks 4 and 5; the antitragus from hyoid hillock 6. Darwin's tubercle appears at 



