DEVELOPMENT OF THE NOSE. 95 



early distinguishable as slight protuberances upon the margins of the shallow cleft- 

 like depression which is all that represents the meatus at this stage (fig. 110, A). But 

 as the body wall becomes thicker, the cleft becomes deepened and more tubular, and 

 the protuberances upon the mandibular and hyoidean arches become gradually so 

 transformed and arranged around the external orifice as to be recognizable as the 

 several parts of the future pinna. The transformations may readily be understood 

 from the study of the accompanying series of sketches from His, which show these 

 parts in gradually advancing stages in the human embryo (fig. 109). 



DEVELOPMENT OF THE NOSE. 



The olfactory organ arises in all vertebrates at an early period of embryonic life 

 as a depression of external epiblast (olfactory pit) on either side of the fore-brain. 

 The epiblast in this region becomes thickened, forming an olfactory area, and a de- 



Fig. 110. PROFILE VIEW OF THE HEAD OF A 



HUMAN EMBRYO OF NKARLY FOUR WEEKS. (His.) Fig. 111. HEAD OF AN EMBRYO MORE ADVANCED 



IN DEVELOPMENT THAN THAT SHOWN IN FIG. 



olf, oltactory depression passing posteriorly 110, FROM BEFORE. (His.) 



into a deep pit, the rudiment of Jacobson's 



organ ; mz, maxillary process ; mn, mandibular pr.fflob, globular extremity of the mesial nasal 

 arch ; hy, hyoidean arch ; ftr 1 , 5?' 2 , first and process. The other letters as in fig. 111. 

 sscond branchial arches. 



pression then forms in this area surrounded by a raised margin (figs. 96, 97, olf). 

 The depression soon appears pyriform, the smaller end extending as a groove towards 

 the stomodoeum or buccal invagination (see fig. 110, olf) ; near this end a special 

 pit is early visible, and becomes developed into Jacobson's organ. 



The thickened boundaries of each olfactory pit and groove are formed by the so- 

 called mesial and lateral nasal processes (figs. Ill, 112). The mesial nasal processes 

 are united at their base by a depressed median part of the fronto-nasal process, but 

 are at first separated below, where they terminate in distinct tubercles, termed by 

 His the globular processes. As development proceeds they extend backwards along 

 the roof of the embryonic mouth, forming the nasal lamina. Eventually the 

 globular processes coalesce in the middle line to form the intermaxillary process and 

 the middle part of the lip, while from the depressed surface between them the 

 lower part of the nasal septum and the philtrum are formed, and by a coalescence of 

 the nasal laminae the rest of the nasal septum is produced. In rodents a notch leads 

 from the nasal septum through the upper lip to the mouth, and represents an im- 

 perfect union of the globular processes. 



Above the depressed surface just referred to, is a triangular part of the fronto- 

 nasal process which forms an angle with it. This angle eventually becomes the 



