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and what now causes other cells of the same 

 primary outer layer to become a line of olfactory 

 nerve cells, attached to the sense organ and arising 

 from a nucleus in the central nervous system. 

 Experimental embryology and physiology of the 

 future may reveal some of the chemical changes 

 involved, as the generalized ectoderm cell differ- 

 entiates into the specialized one capable of only 

 one class of reactions; but this will only widen our 

 knowledge of the bewildering complexity of the 

 single fertilized egg cell, which divides and sub- 

 divides so as to give rise to the olfactory organs as 

 well as to all other parts of the body. 



Meanwhile, as stated above, the main tran- 

 sitional stages in the evolution of the nose from 

 fish to man are fairly well understood, and are well 

 described in Keith's Morphology and Embryology. 

 First the olfactory sac becomes folded up, and in 

 sharks a groove (Fig. 66A) extends downward 

 toward the corner of the mouth. Second, in the 

 lung-fishes this lower extension of the sac has 

 worked its way inside the mouth and there are 

 thus two openings, a nostril on the outside and 

 an internal narial opening in the roof of the mouth. 

 Third, both in the air-breathing fishes and the 



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