AUTHOR S ABSTRACT OP THIS PAPER ISSCTED 

 BY THE BIBLIOGRAPHIC SERVICE, MAY 1 



VERTEBRATE CEPHALOGENESIS 



IV. TRANSFORMATION OF THE ANTERIOR END OF THE HEAD, 

 RESULTING IN THE FORMATION OF THE 'nOSE' 



HOWARD AYERS 



TWENTY-SIX FIGURES 



CONTENTS 



Introductory 323 



1 . Amphioxus 324 



2. Ammocoetes and Petromyzon 329 



3. Bdellostoma 333 



4. .Chimpanzee and man 336 



5. The anterior cranial nerves 336 



6. Comments on function 340 



INTRODUCTORY 



From the investigations of. many anatomists we have come to 

 recognize, first, that the jaw apparatus (including the tongue 

 and mouth) is a mechanism built up out of old-time head carti- 

 lages and newer elements derived from the gills, which has been 

 added to the primitive vertebrate head — in fact, a mechanism of 

 the trigeminus. Second, that the whole auditory apparatus is a 

 mechanism built up out of surface sense organs sunk below the 

 surface, together with structures derived from the gills. Third, 

 that the eyes and adnexa are the end result of the outpushing 

 from the brain of two hollow globes, whose walls are made up of 

 the pigmented light sensitive cells of that part of the central 

 nervous axis behind the lamina terminalis, with numerous asso- 

 ciated parts. These three prominent organ systems have been 

 quite thoroughly worked out and their component parts traced 

 back to their origins. In speaking thus of the evolution of these 

 three prominent additions to the head, we include those other 

 necessary structures, as blood, nerve and lymph supply, and the 

 muscles, connective- tissue parts and skin, which go to correlate 



323 



