634 Jacob Parsons Schaeffer. 



to 60 days will indicate the beginning formation of the compli- 

 cated nature of the meatus nasi medius — the most complex of 

 all the meatuses. This change in the middle meatus is heralded 

 by the appearance of a somewhat crescentic-shaped fold of mesen- 

 chyme and mucous membrane, set off by a slightly earlier groov- 

 ing immediately superior to the fold. The latter has its free and 

 rather sharp border directed superiorly and dorsally, and is the 

 anlage of the processus uncinatus. The furrow immediately supe- 

 rior to the fold is the primitive infundibulum ethmoidale (fig. 18). 

 It will be noticed that the furrow communicates rather freely 

 with the meatus nasi medius through a rather widely open cleft — 

 the hiatus semilunaris. 



If now we examine a frontal section of a somewhat later embryo 

 we will notice that the infundibulum ethmoidale early tends to 

 develop beyond its primitive limits, and to pouch towards the 

 ventral and superior portion of the meatus nasi medius, here 

 ending blindly. This early, blind, ventral and superior exten- 

 sion of the infundibulum ethmoidale has been erroneously con- 

 sidered by some writers as the primitive sinus maxillaris (fig. 32). 

 Of the bulla ethmoidalis there is nothing to be seen at this time. 



About the seventieth day there is a slight, sometimes a compara- 

 tively extensive, pouching or evagination of the mucous membrane 

 from the depth of the infundibulum ethmoidale — thus establish- 

 ing the anlage of the sinus maxillaris. Shortly after this we have 

 the first evidences of the bulla ethmoidalis appearing superior and 

 lateral to the processus uncinatus — this in the form of very low 

 accessory folds or conchae. By the beginning of the fourth month 

 of fetal life the folds of the bulla are fairly well formed. At the 

 end of the fourth month the folds with the bordering accessory 

 furrows are in many cases well outlined (fig. 18). From some of 

 the accessory furrows the so-called middle group of ethmoidal 

 cells develop, and in a sense the former are the anlages of the 

 latter structures. At a comparatively early stage we have evi- 

 dences of a beginning extension of the middle meatus in a ventral 

 and superior direction. This is the anlage of the recess, which 

 Killian in later fetuses terms the 'Recessus frontalis' and it is 

 in reality the first step in the formation of the sinus frontalis 



