64 J. B. Johnston. 
are demonstrated with the greatest ease. ‘The teeth are formed from 
ectoderm which migrates in from the borders of the mouth. The 
tooth germs and the teeth when well formed lie beneath the ento- 
dermal lining of the mouth. The teeth pierce the entoderm at about 
the time that the taste buds are becoming clearly recognizable. Up 
to this time the entoderm forms a perfectly continuous lining of the 
whole mouth. It is broken now only by the points of the teeth piere- 
ing it. Up to the stage described no ectoderm cells have gained a 
place in the lining membrane in any part of the mouth. The taste 
buds are well formed by this stage, their formation from entoderm 
cells is directly observed in Amblystoma, and they have no relation 
to ectoderm. 
Whether in later stages ectodermal cells of the dental ridges rise . 
to the surface around the teeth and form the covering of the gums 
or of a larger surface in the mouth I have not yet studied. It may be 
difficult to determine this point, as the entoderm in the mouth loses 
its yolk rapidly in the few days following the last stage described. 
The innervation of the mouth suggests the probability that some part 
of it is lined by ectoderm. The anterior part of the roof of the mouth, 
in front of the vomers, receives cutaneous components from the 
ophthalmicus profundus along with visceral sensory components from 
the n. palatinus VII (Coghill, 1902). It seems probable that the 
taste buds are innervated by the fibers of the palatine nerve, as they 
are always innervated by branches of the communis VII. Whether 
the trigeminal fibers invade purely entodermal territory or whether 
ectodermal cells insinuate themselves into the lining of the mouth 
in this region is unknown. This question, however, has no relation 
to the question of the origin of taste buds since it is certain that no 
ectoderm enters into the lining of the mouth until long after the taste 
buds are formed, if at all. The fact that the trigeminus helps to 
innervate the mouth, especially in mammals and man, has given 
tacit support to the idea that the taste buds are of ectodermal origin. 
The reasoning involved in this assumption is illogical, however, since 
the taste buds are themselves innervated by the facial nerve which is 
strictly a nerve related to entodermal surfaces in all vertebrates 
except those fishes in which taste buds spread into the outer skin. 
