Lateral Wall of the Cavum Nasi in Man. 621 



quently, however, the later extensions of the lateral nasal processes, 

 medially and ventrally above the maxillary processes, meet 

 and fuse with the medial nasal processes, to form the imme- 

 diate inferior boundaries of the nasal pits (fig. 1). His has also 

 pointed out, in case of arrested development in this region, that 

 at times the lateral and medial nasal processes fuse, but that the 

 maxillary process, on one or both sides, remains short and does 

 not develop sufficiently to fuse with the medial nasal process as 

 it normally does. In such cases the nasal pits are closed in below 

 and are separated from the oral cavity, yet clefts exist between 

 the medial nasal and the maxillary processes. 



Fusion also takes place laterally between the maxillary and 

 lateral nasal processes, and from the strands of ectodermal tissue 

 caught between the edges of the coalescing processes, the nasolac- 

 rimal ducts develop, i.e., the ectodermal strands acquire lumina. 

 These ducts will again be referred to in connection with their 

 apertures on the lateral nasal walls. The ducts usually establish 

 communications with the inferior nasal meatuses at about term. 

 This may however be longer delayed. 



For some time the lines of fusion of the maxillary and the lateral 

 nasal processes with the medial nasal processes are represented 

 by strands of ectodermal tissue. These ectodermal fusion-lines 

 soon disappear ventrally and are replaced by indifferent mesen- 

 chyme, i.e., the mesenchymal tissue of the maxillary and the 

 lateral nasal processes becomes continuous with that of the medial 

 nasal processes. Fusion in this manner becomes permanent and 

 absolute (fig. 3). Farther dorsally the ectodermal tissue does not 

 wholly disappear, but strands of the latter tissue remain between 

 the abutting processes. In these positions the primitive choanse 

 ultimately become established (figs. 2 and 4). 



The 'primitive nasal fossae 



By this time (35-day embryo) the nasal pits have deepened 

 dorsally, and more or less superiorly and inferiorly. The olfac- 

 tory organ is now represented by two blind pouches lying in the 

 mesenchymal tissue above the oral cavity. The pits have now 



