616 Jacob Parsons Schaeffer. 



II The Formation of the Nasal Fossae 

 The nasal anlage 



The nasal anlage apparently establishes itself about the third 

 week of embryonal life as localized thickenings of the ectoderm. 

 Kallius and Mihalkovics place the earliest trace of the nasal anlage 

 at the beginning of the third week, and Bryce at the end of the 

 third week of embryonal life. Mrs. Gage refers to the nasal 

 epithelium in a ''three weeks human embryo." 



These thickened ectodermal or nasal areas are situated on both 

 sides of the outer surface of the wall of the fore-brain, just supe- 

 rior to the primitive oral fossa. Whether the nasal areas of the 

 human embryo are at first connected, and later undergo division 

 into two distinctly separate halves, is not definitely known. The 

 H-shaped nasal area of Mrs. Gage's 21-day embryo seems signifi- 

 cant. She figures two lateral plates connected by an intervening 

 bar of apparently similarly thickened ectoderm. Minot says: 

 "It is possible that more exact observation will show that in all 

 vertebrates there is at first a single plate, which is early divided." 

 Doubtless this will remain a mooted point in man until a sufficient 

 number of human embryos of the proper age fall into the hands of 

 different observers. 



Bedford found the thickness of the olfactory areas in swine 

 embryos of 5 mm. length to be 0.075 mm. Kallius found that, 

 while the general ectodermal thickness was 0.01 mm., the thickened 

 nasal areas measured from 0.044 to 0.060 mm. 



The nasal pits 



During the fourth week the nasal areas become depressed — • 

 according to Mihalkovics at the end of the third week. This 

 depression is not due to an invagination of the nasal areas, but 

 it is passive and is caused by an increase in the thickness of the 

 surrounding mesoderm. These ridges of thickened mesoderm 

 around the nasal areas push the ectoderm into relief. In this 

 manner each nasal area becomes surrounded by a fold,* and is 



