244 '™^ EVOLtJTIOK OF MAIT. 



original separation of the nasal groove from the mouth 

 groove is, however, soon interrupted, for the frontal procesa 

 (Fig. 235, st, Rathke's " Nasenfortsatz der Stirnwand ") 

 is immediately formed above the mouth groove. Right and 

 left the edges of this process project in the form of two 

 lateral processes : these are the inner nasal processes, or 

 nasal flaps (Fig. 235, in). On each side, opposite to these 

 rises a parallel ridge between the eye and the nasal groove. 

 These ridges are the outer nasal processes (Rathke's "Nasen- 

 dacher," Fig. 235, an). Between the inner and outer 

 nasal process a channel-like depression thus extends on 

 each side from the nose groove toward the mouth groove 

 (m), and this channel is, of course, the same nasal furrow 

 or channel which we found in the Shark (Fig. 231, r). As 

 the two parallel edges of the inner and the outer nasal 

 processes bend towards each other and coalesce above the 

 nasal channel, the latter becomes a small tube — the primitive 

 " nasal canal." In this stage of its Ontogeny, therefore, the 

 nose of Man and of all other Amnion Animals consists of 

 two small narrow tubes — the " nasal canals " — leading from 

 the outer surface of the frontal skin into the simple pri- 

 mitive mouth-cavity. This transient condition resembles 

 the permanent condition of the nose in Dipneusta and 

 Amphibia. (Cf. Plate I., Frontispiece, with explanation.) 



Specially significant in the modification of the open nasal 

 channel into the closed nasal canal, is a plug-shaped forma- 

 tion, which extends from below up to the lower extremities 

 of both the nasal processes on each side, and unites with 

 them. This is the upper jaw process (Figs. 232, o, 236, o, 

 Plate I., o). Below the mouth groove lie the gill arches, 

 which are separated from one another by the gill openings 



