THE MOUTH-SKELETON. 33! 



We have already considered this remarkable formation, and 

 will only call attention once more to the interesting fact that 

 the human middle and external ear is the last remnant 

 of the gill-opening of a Fish. The gill-arches, also, which 

 separate the gill-openings, develop into very various parts. 

 In Fishes they remain permanently as gill-arches, carrying 

 the respiratory gill -tufts ; so also in the lowest Amphibia ; 

 but in the higher Amphibia they undergo various modifica- 

 tions in the course of development, and in all the three 

 higher vertebrate classes, thus also in Man, the tongue-bone 

 (os hyoides) and the bonelets of the ear originate from the 

 gill-arches. (Of. Plates VI. and VII.) 



From the first gill-arch, from the centre of the inner 

 surface of which the muscular tongue grows, proceeds 

 the rudimentary jaw-skeleton ; the upper and lower jaws 

 which enclose the cavity of the mouth and carry the 

 teeth. The Acrania and Monorhina are entirely destitute 

 of these important parts. They first appear in the genuine 

 Fishes, and have been transmitted by these to the higher 

 Vertebrates. The original formation of the human mouth- 

 skeleton, of the upper and lower jaws, can thus be traced 

 back to the earliest Fishes, from which we have inherited 

 them. The teeth originate from the outer skin-covering 

 which covers the jaws ; for, as the formation of the whole 

 mouth-cavity takes place from the outer germ-layer, the teeth 

 must, of course, also have developed originally from the skin- 

 layer. This can be actually proved by close microscopic 

 examination of the most delicate structural features of the 

 teeth. The scales of Fishes, especially of Sharks, are, in 

 this respect, exactly similar to their teeth (Fig. 288). Thua 

 the human teeth, in their earliest origin, are modified fish- 



