20 THE NASAL ACCESSORY SINUSES IN MAN 



intervening ectoderm is resorbed, and the processes become 

 united by mesoderm, which forms the floor of the primitive 

 anterior nares. In the fifth week, or shortly thereafter, by 

 the partial resorption of ectodermal cells filling the remain- 

 ing interspaces between the median and lateral nasal proc- 

 esses, there is formed behind the os intermaxillare the 

 primitive ductus nasopharyngeus. Posteriorly, there per- 

 sist for a short while the membranse bucconasales, which 

 break through and form the primitive choanae at a time 

 somewhere be ween the twenty-eighth and fortieth day the 

 observations of the different embryologists varying to that 

 extent. 



The basal part of the primordial cranium develops into 

 chondrocranium. Laterally, the basal plates develop as the 

 periotic capsules; ventrally they form the mesethmoid 

 plate (from the lower portion of which the vomer later 

 develops), and anterolaterally they enter into the formation 

 of the nasal capsule. In the second month the nasal capsule 

 becomes clearly differentiated from other mesoderm and 

 shows beginning cartilaginous development. 



The palatal ridges appear on the medial sides of the 

 maxillary processes from the forty-fifth to forty-eighth day 

 of embryonal life (J. P. Schaeffer), and by their approxima- 

 tion the palate is formed. 



Concerning the early development of the conchse, Schaeffer 

 records in his very thorough studies of the embryologic 

 development of the lateral nasal wall that the concha 

 nasalis inferior appears in embryos of thirty-eight to forty 

 days as a bulging of the inferior portion of the lateral nasal 

 wall immediately superior to the portion from which the 

 palatal processes develop. From the fortieth to the forty- 



