Lateral Wall of the Cavum Nasi in Man. 699 



must not be sought in the vertical portion of the frontal bone but 

 in the superior and ventral portion of the meatus medius, i.e., 

 in the frontal recess. The developing frontal sinus may not pro- 

 gress farther than the horizontal portion of the frontal bone. The 

 different views held on the presence and absence of the frontal 

 sinus is doubtless due to differences of opinion as to what should 

 be called a frontal sinus, and how far the development must pro- 

 gress towards the frontal region before the cell has reached the 

 dignity of a frontal sinus. 



VI Summary 



1. The nasal anlage establishes itself about the third week 

 of embryonal life as localized thickenings of the ectoderm, located 

 on both sides of the outer surface of the wall of the fore-brain, 

 immediately superior to the primitive oral fossa. 



2. During the fourth week the nasal areas become passively 

 depressed, due to an increase in the thickness of the surrounding 

 mesenchyme. In this manner the nasal pits become formed. 



3. For some time the nasal pits communicate freely with the 

 primitive oral fossa. The maxillary and the lateral nasal processes 

 abut and fuse with the medial nasal processes, thus separating the 

 nasal pits from the oral fossa. 



4. For some time the lines of fusion of the maxillary and the 

 lateral nasal processes with the medial nasal processes are repre- 

 sented by strands of ectodermal tissue. These ectodermal fusion- 

 lines soon disappear ventrally and are replaced by mesenchyme; 

 i.e., the mesenchymal tissue of the maxillary and lateral nasal 

 processes becomes continuous with that of the medial nasal 

 processes. 



5. Farther dorsally the ectodermal tissue between the fused 

 maxillary and medial nasal processes does not wholly disappear, 

 but strands of ectodermal tissue remain at the points of fusion. 

 This ectodermal tissue thins out and ultimately ruptures, thus 

 establishing the primitive choanae. 



6. The nasal pits deepen dorsally and superiorly, and in a 

 35-day embryo the olfactory organ is represented by two blindly 



