114 DEVELOPMENT OF THE AURICLE IN THE HUMAN EMBRYO. 



it will be necessary to study separately each part of the body and establish the 

 normal sequence of differentiation, region by region. 



What was done by Spaulding for the external genitalia I have endeavored to do 

 for the branchial region, and it is my purpose in the following pages to outline what 

 seem to be the significant morphological features in the transformation of the 

 tissues in the neighborhood of the first gill-cleft into the definitive auricle. 



HISTORICAL. 



Most of the investigators who have published accounts of the development 

 of the auricle have shown a lively interest in the branchial hillocks and have placed 

 great emphasis on them as the essential factors in the acquirement of the final form 

 of the auricle. I am of the opinion that too much importance has been attributed 

 to these hillocks, and that the auricle, instead of being a composite structure 

 the fused product of a group of separate and discrete masses comes into existence 

 as an intact and continuous primordium, which, by the ordinary processes of dif- 

 ferentiation, gradually becomes elaborated into its final form. It arises,' for the 

 most part, from the mesenchymal cells of the hyoid bar; the overlying ectoderm, 

 also, may play an important role in its determination. It is possible that it is 

 entirely of hyoid origin and that the mandibular elements are nothing more than 

 the product of cells that have migrated forward into that region. In support of 

 this idea is the fact that the mandibular parts, when first seen, are mostly in the 

 deeper levels. However that may be, as soon as one can begin to outline the con- 

 densed tissues constituting its primordium, the whole auricle is continuous and 

 exhibits the essential contours of the mature structure. 



Before entering into this subject more fully, it might be well to outline the 

 principal steps in our present knowledge regarding the development of the auricle. 

 To make the history brief, condensed abstracts of the more significant observa- 

 tions will be given in chronological order, as far as I have been able to follow them. 



Moldenhauer (1877), in a careful study of the development of the middle and 

 external ear of the chick, discovers the occurrence of two pahs of hillocks on the 

 first and second branchial arches, which he terms colliculi branchiales externi. He 

 regards these as connected with the development of the external auditory meatus, 

 the tragus being derived from the first arch and the anti-tragus from the second 

 arch. They are present on the sixth and seventh daj^s of incubation, and on the 

 eighth day they become transformed into the definitive parts of the meatus. Think- 

 ing of the head as erect, with its longitudinal axis in the vertical plane, the author 

 speaks (p. 118) of the hillocks in front of the first gill-cleft as "superior" and those 

 behind the first gill-cleft as "inferior." The ventral pair he calls "anterior " and the 

 dorsal pah- "posterior." Thus, the hillocks of the mandibular bar become, respec- 

 tively, colliculus posterior superior and colliculus anterior superior, and the hillocks 

 of the hyoid bar become colliculus posterior inferior and colliculus anterior inferior. 



His (1882), in describing the external form of human embryos between 12 

 and 30 mm. long, briefly mentions the occurrence of branchial hillocks around the 

 first gill-cleft, similar to those found by Moldenhauer in the chick. Instead of four, 



