242 THE EVOLUTION OF 3VIAN. 



cavity, which latter was developed independently of the 

 groove. In the Dipneusta and the lower Amphibia the 

 internal opening of the nasal canal lies well forward (behind 

 the lips) ; in the higher Amphibia it lies further back. In 

 the three highest vertebrate classes, the Amniota, the 

 primary mouth-cavity is separated by the formation of the 

 horizontal palate roof into two perfectly distinct cavities, 

 the superior (or secondary) nasal cavity, and the inferior 

 (or secondaiy) mouth-cavity. The nasal cavity is also 

 separated by the vertical partition into two distinct halves, 

 into a right and a left nasal cavity. 



Comparative Anatomy thus still shows us simultaneously, 

 in the ascending series of the double-nostrilled Vertebrates, 

 from Fishes up to Man, all the various stages of develop- 

 ment of the nose which the very highly developed olfactory 

 organ of the higher Mammals has passed through succes- 

 sively in the different periods of its tribal history. The 

 first rudiment of the organ of smell in the embryo of Man 

 and in that of all the higher Mammals, makes its appearance 

 in the same entirely simple form which is retained throughout 

 life by the nose of Fishes. At a very early stage, and 

 while no trace of the characteristic facial structure of Man 

 is yet visible, a pair of small grooves appear on the front 

 of the head, and before the primitive mouth-cavity ; these 

 were first discovered by Baer, and by him properly enough 

 named " olfactory grooves " (" Biechgruben," Figs. 232, n, 

 233, n). These primitive nasal grooves are quite separate 

 from the primitive mouth-cavity, or mouth indentation, 

 which, as we found, likewise makes its appearance as a 

 groove-like indentation of the external skin-covering, in 

 front of the blind anterior extremity of the intestinal canal. 



