DEVELOPMENT OF THE AURICLE IN THE HUMAN EMBRYO. 



135 



up of those condensed parts of the hyoid and mandibular bars that constitute 

 the auricular primordium. The widening of the angular fossa can be partly 

 accounted for by the spreading apart of the auricular rim through the growth of the 

 tissues composing its floor (closure plate), in which can be seen forming the head of 

 Meckel's cartilage. 



In embryos up to 16 or 18 mm. the condensed mesenchyme forming the pri- 

 mordium of the auricle is fairly uniform in appearance, but at about this time one 

 can begin to see clearly the auricular cartilage separating itself from the less dense 

 tissue as a lamina of precartilage cells. As soon as it can be recognized, this lamina 

 is found to be folded in a manner essentially like that of the adult cartilage. The 

 scapha-helix stands out prominently, the free edge of the helix remaining in contact 

 with the ectoderm. The anthelix is also indicated almost from the first, whereas 



21 



3Zr 



43 r 



Fig. 6. Fio. 7. 



Fig. 6. Lateral views of left auricular cartilage, taken from reconstructions of human embryos of the Carnegie Collection: 



No. 460 (21 mm.), No. 417 (32 mm.), No. 886 (43 mm.). X14. 

 Fig. 7. Reconstruction of left auricular cartilage of a 50 mm. fetus (No. 84, Carnegie Collection). X14. A model of the 



external form of the auricle was made, in conjunction with the cartilage, to give the topographical relations. 



The edge of the helix in contact with the ectoderm is indicated by cross-lines. Compare with figures 35 and 38. 



the concha is less sharply defined, and it is not until the embryo has reached a 

 length of 40 to 50 mm. that the cartilage may be considered to have acquired its 

 definitive adult form. In this respect, however, it is much in advance of the sur- 

 face form of the auricle. It is quite evident that the folding of the cartilage is not 

 produced mechanically by resistance to its expansion on the part of the ectoderm, as 

 has been maintained; the surrounding tissues are loose enough to make folding 

 unnecessary. Furthermore, the folding is relatively as great at first, when the 

 cartilage is small, as it is in the later stages. The auricular cartilage clearly acquires 

 its form with all the precision and individuality shown by the other cartilaginous 

 parts of the body. 



The transition from the arrangement of typical branchial bars to an auricle 

 of primitive type takes place during the period represented by embryos from 4 to 

 16 mm. By that time the nodular elevations caused by the hillocks are for the most 

 part smoothed out, and we find the angular fossa inclosed by a rounded border 



