134 DEVELOPMENT OF THE AURICLE IN THE HUMAN EMBRYO. 



branchial cleft. These hillocks, since the early paper of His, have received much 

 attention and have been variously designated, and descriptions have been given 

 of how, by their unequal growth and subsequent coalescence, the eventual auricle 

 comes into existence. The hillocks have been so interpreted in spite of the fact 

 that it was known that they present much the same appearance in mammals that 

 have, in the adult stage, very different types of ears from those of man, and that 

 they are present even in birds and reptiles, which never acquire a distinct auricle. 



From what has been stated in the foregoing pages one is forced to the conclusion 

 that the hillocks, as such, are of a transitory character and are incidental, rather 

 than fundamental, to the development of the auricle. Probably of more signifi- 

 cance, as far as the derivatives of these parts are concerned, is the division of the 

 mandibular and hyoid bars into ventral and dorsal segments, the closure mechanism 

 being derived from the former, the articular mechanism and scapha helix from the 

 latter. 



The essential histological change which inaugurates the formation of the 

 auricle (embryos between 4 and 14 mm.) consists of a proliferation and conden- 

 sation of the mesenchyme. The mesenchymal change is accompanied by evidences 

 of marked activity of the ectoderm over the whole auricular area. The deeper 

 layer of ectoderm cells enlarge, proliferate, pile up two or three cells thick, and at the 

 same time develop elongated, cylindrical bodies or processes which project toward 

 the abutting mesenchyme, thus forming a narrow, clear cytoplasmic band at 

 the mesenchymal junction. Directly beneath the ectoderm the mesenchymal 

 cells are crowded into a compact fine of proliferating elements from which great 

 numbers of cells can be seen streaming into the deeper levels. The condensation 

 of the mesenchyme is thus most intense at the ectoderm and gradually becomes 

 less marked in the looser tissues of the central part of the bar. This phenomenon 

 of ectodermal and mesodermal activity takes place over the whole surface of the 

 hyoid bar, and in a less degree over the posterior half of the mandibular bar, in 

 which the condensed mesenchyme soon becomes localized in the deeper layers. 

 It is more prominent in those parts where the auricular cartilage arises, and almost 

 from the first gives the outlines of the cartilage in its primitive form, so that we may 

 speak of it as the primordium of the auricle. The relation of the branchial hillocks 

 to the auricular primordium appears to be that they are merely foci in which the 

 mesenchymal proliferation is temporarily most rapid; they do not represent the 

 entire auricular primordium. This is particularly evident in hillocks 4 and 5. 

 In the hillocks of the mandibular bar (1, 2, 3) the mesenchyme is not so compact, 

 although there also it is in active proliferation. 



The proliferation and condensation of the branchial mesenchyme constituting 

 the primordium of the auricle and the rearrangement of the mesenchyme where 

 the condensation is less marked produce a change in the surface form of the gill- 

 bars. The narrow hyoid cleft thereby becomes converted into a broad fossa angu- 

 laris. The width of the fossa is increased by a relative sinking in of those portions 

 of the bars adjacent to the cleft. This depression is not so much an actual sinking 

 in as an elevation of the surrounding parts, especially of the auricular rim, made 



