246 THE EVOLUTION OF MAN. 



adhere in the middle line, the result is a permanent longi- 

 tudinal cleft, through which there is an open passage from 

 the mouth-cavity directly into the nasal cavity. The so- 



Fig. 237. — Diagrammatic transverse section 

 through the mouth and nose cavity. While the 

 palate-plates (p) separate the original mouth-cavity 

 into the lower secondaiy mouth-cavity (m) and the 

 upper nasal cavity, the latter is parted by the ver- 

 tical partition wall of the nose (e) into two distinct 

 halves (n, n). (After Gegenbaur.) 



called " wolf's jaws " are thus caused. The " hare-lip " and 

 "split lip" is a slighter degree of this arrested develop- 

 ment. 116 



Simultaneously with the horizontal partition of the 

 palate roof, a vertical wall by which the single nasal cavity 

 is divided into two, a right and a left cavity, develops 

 (Fig. 237, n, n). This vertical partition of the nose (e) is 

 formed by the middle part of the frontal process : above 

 this gives rise by ossification to the vertical lamella of the 

 sieve bone (cubiform plate), and below the great vertical 

 bony partition wall — the " plough-share ' ; (vomer), and in 

 front to the twixt-jaw (os intermaxillare). Goethe was 

 the first to show that in Man, just as in all the other Skulled 

 Animals, the twixt-jaw appears as an independent bone 

 between the two halves of the upper jaw. The vertical 

 partition wall of the nose finally coalesces with the horizontal 

 palate roof. The two nasal cavities are now as entirely 

 separate from one another as from the secondary mouth- 

 cavity. These three cavities open, however, at the back 

 into the pharnyx, or jaw-cavity. 



