188 J. PARSONS SCHAEFFER 



difficulty. The narrow slit extends in a cephalo-caudal direction 

 and it contrasts somewhat with the wide slit-like type represented 

 in fig. 5. 



Rarely we find an anomalous type of ostium represented in 

 fig. 7. The ostium is located on a raised or nipple-like projec- 

 tion of the nasal mucous membrane. 



The ostium represented in fig. 4 is also unusual. It is an ex- 

 tremely small, circular opening located immediately caudal to 

 the attached border of the concha nasalis inferior. The common 

 type of ostium for this position is the wide, unguarded, open- 

 mouthed type. 



The other figures represent variations of the principal types 

 already referred to. The normal anatomic types, since they occur 

 so frequently in the series of specimens studied, are represented in 

 figs. 2, 5, 6, 9 and 10. Figs. 4 and 7 are illustrations of anomalous 

 ostia, occurring very infrequently in the series. Figs. 1, 3, 8 and 

 11 are variations of the normal anatomic types. 



GENETIC SIGNIFICANCE OF THE SEVERAL TYPES OF OSTIA 

 NASOLACPJMALIA 



It is well known from the work of many observers that the naso- 

 lacrimal passages have their anlage in a thickening of the rete 

 mucosum of the epidermis, along the floor of the rudimentary 

 naso-optic furrow. This solid strand of epidermal cells ultimately 

 becomes entirely isolated from the surface (in man) and for some 

 time is wholly encompassed by mesenchymal cells. 2 From the 

 ocular end of the isolated epidermal strand of cells develop two 

 sprouts which become the ductus lacrimales (superior and inferior). 

 There is also a sprouting from the nasal end of the strand of cells 

 which in time grows sufficiently to establish coalescence with the 

 mucous membrane of the lateral wall of the meatus nasi inferior. 

 The anlages of the nasolacrimal passages later become canalized.' 



The manner of coalescence of the strand of epidermal cells 

 with the nasal mucous membrane especially concerns us with 



2 J. Parsons Schaeffer, The genesis and development of the nasolacrimal passages 

 in man. Amer. Jour. Anat., vol. 13, no. 1, 1912. 



3 Loc. cit. 



