EMBRYOLOGIC CONSIDERATIONS 



During the third week of embryonal life there is evidence 

 of beginning development of the nasal areas as shown by 

 increasing thickness of the ectoderm on the anterolateral 

 portions of the forebrain. At the end of the third week, or 

 during the fourth week, the nasal area appears as a depression 

 which is brought about by the increased thickness of the 

 surrounding mesenchyme. In the mesenchyme investing 

 the anterior portion of the notochord is the first appearance 

 of the primordial cranium; this then extends dorsally to 

 inclose the anterior portion of the medullary canal, which 

 will later become the cerebral part of the central nervous 

 system. The primitive nasal capsule develops as a part of 

 the primordial cranium. From that part which extends 

 forward beyond the anterior portion of the notochord a core 

 is formed for the frontonasal process a relatively broad 

 mass of tissue separating the nasal pits. The nasal pits are 

 symmetric depressions, at first pyriform in outline, with 

 the small ends toward the primitive mouth cavity. As the 

 processus globularis on the lateral portion of the medial 

 nasal process approximates the lateral nasal and maxillary 

 processes, there is a deepening of the primitive nasal fossae 

 and a change in their form, which becomes oval and bordered 

 by broad folds. The broad median process separating the 

 depressions later becomes narrowed and forms the septum 

 nasi. By the approximation of the nasal processes inferiorly, 

 their ectodermal coverings are brought into contact. The 



19 



