700 Jacob Parsons Schaeffer. 



ending pouches lying in the mesenchymal tissue above the oral 

 cavity. These blind pouches may be termed the primitive nasal 

 fossae. 



7. The dorsal extension of the blind primitive nasal fossae 

 continues until the ectoderm of the nasal fossae meets the ecto- 

 derm of the oral cavity. We have now in these positions merely 

 thin membranes composed of two layers of abutting epithelium — 

 nasal and oral — separating the dorsal portions of the primitive 

 nasal fossae from the oral cavity. 



8. These membranes — the membranae bucconasales — become 

 attenuated and thinned out, ultimately resulting in rupture. In 

 this manner the primitive choanae are established, and again 

 a communication between the nasal fossae and the oral cavit}^ 

 The primitive choanae are established approximately from the 

 35th to the 38th day of embryonal life. 



9. When first formed the nares communicate freely with the 

 exterior, but shortly afterwards, in many cases, they become 

 plugged by an overgrowth of epithelium. This plugging may be 

 complete or more or less fenestrated in character. 



10. With the formation of the primitive choanae we have also 

 established the primitive palate, i.e., the portion of the roof of 

 the primitive oral cavity extending from the primitive choanae 

 to the nares. 



11. The palatal processes; by fusing along their opposed edges 

 in the median plane ; establish the definitive palate. In the forma- 

 tion of the definitive palate the nasal fossae appropriate a consider- 

 able portion of the primitive oral cavitj' . 



12. The nasal cavity which is early divided anteriorly by the 

 medial nasal processes, becomes divided into fossae farther dor- 

 sally by the growth and fusion of the nasal septum with the 

 mid-palate line. This division of the nasal cavity into fossae 

 takes place from before backwards. 



13. Coincidently with the formation of the definitive palate 

 the primitive choanae elongate and ultimately come to occupy 

 the position of, and thus become, the permanent choanae. 



14. The lateral nasal wall is at first extremely simple, and it 

 presents no evidence of its later complexity. 



