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DEVELOPMENT OF THE AURICLE IN THE HUMAN EMBRYO. 



thick, beneath which is a layer of closely packed cuboidal cells with large round 

 nuclei. It is this deeper layer that appears to be chiefly involved in the process of 

 proliferation. In slightly older specimens it becomes several cells thick, and in 

 some specimens one finds, at the point where it abuts against the mesoderm, a clear 

 white line consisting of the elongated cell-bodies of the proliferating ectoderm. 

 The changes in the ectoderm are most marked over the areas where the condensa- 



Fig. 5. A diagrammatic interpretation of figures 13 to 27 (plates 2 and 3), showing the advent and disappearance of the 

 branchial hillocks and the coincident changes in the mandibular and hyoid bars. These stages cover the period 

 of transition from a state of simple branchial bars to the establishment of the primitive auricle. The hillocks 

 are interpreted by the author as foci of more active proliferation of the condensed mesenchymal primordium 

 of the auricle. A to C, embryos 5 to 11 mm.; D to G, embryos 13 to 14 mm.; H to K, embryos 15 to 18 mm.; 

 L to O, embryos 18 to 33 mm. Varying magnifications were adopted so as to bring the structures to about 

 the same size. 



tion of mesenchyme is greatest and more marked over the hyoid bar (hillocks 

 4 to 6) than over the mandibular bar (hillocks 1, 2, 3). The whole auricular region, 

 however, exhibits this phenomenon and stands out in strong contrast to the adja- 

 cent portions of the head. The evidence of activity on the part of the ectoderm 

 of the auricular region is very striking and appears to be closely related to the 

 changes in the subjacent mesenchyme. In appearance it resembles very much the 

 ectoderm of the arm and leg buds in their earlier stages. I shall refer to this subject 

 later. 



