EMBRYOLOGIC CONSIDERATIONS 25 



third day the ethmoidal fold appears superior and slightly 

 dorsal to the fold representing the concha inferior, and from 

 this ethmoidal fold the ethmoidal conchse are developed as 

 the nasal cavity increases in its supero-inferior diameter. 



Coming now to the conditions shown in the sixty-day 

 embryo (the youngest studied in this series), we find the 

 cartilaginous development in the nasal capsule well ad- 

 vanced. The anterior nares are filled by the masses of 

 epithelial cells (Fig. 1), a condition persisting in some 

 specimens up to one hundred and fifteen days. The lateral 

 nasal wall shows two distinct folds (Fig. 4), the lower being 

 the concha nasalis inferior and the superior one the ethmoidal 

 fold, which is beginning to assume the form of the concha 

 nasalis media. In its posterosuperior portion, the ethmoidal 

 fold shows a further differentiation, indicating the early 

 formation of the concha superior (Fig. 5). The conchse, 

 in their earlier stages of development, do not contain car- 

 tilage, but are folds of mesenchyme covered by nasal epi- 

 thelium. In the sixty-day embryos (Figs. 1-5) the central 

 portion of the concha inferior and the concha media shows 

 condensation of the mesenchyme and its transformation 

 into cartilage. In the concha superior, however, cartilagin- 

 ous development has not yet begun. As the central por- 

 tion of the mesenchyme entering into the formation of the 

 conchaB becomes more dense and is transformed into carti- 

 laginous structures, it appears as an extension of or a pro- 

 jection from the cartilage forming the lateral portion of the 

 nasal capsule. 



As the conchse become more prominent medially, there is 

 also a lateral deepening of the grooves immediately beneath 



