154 
this region great retardation in the development of the posterior 
members of the alternating series occurred’), and they are now 
added from behind for some time after hatching, without regard to 
the dentition to ‚which they belong (Figs. 2 and 3). These posterior 
alternating teeth are also much larger than the anterior ones, but 
retain their alternation in size (Figs. 3 and 4). The retardation 
(of the posterior alternating teeth) which has taken place in the pa- 
latines and the lower jaw is not so great. As I have said, the process 
of tooth displacement and succession is now confined to the anterior 
ends of the tooth bearing regions and is, I believe, gradually under- 
going suppression. Its great restriction is probably due to the dis- 
organisation produced by the lengthening of the incubation period (see 
also later), and as regards the posterior alternating teeth of the maxilla, 
its complete suppression is perhaps directly due to the fact that the 
members of the earlier dentitions are here not developed until long 
after they should have given place to their successors. As regards 
the uniform series, which is of so much importance in the lower jaw, 
and of so much less in the upper (Fig. 4), the more or less prolonged 
period of inactivity of the dental laminae previous to the appearance 
of its earliest members suggests that it is a late acquirement, also 
probably owing its origin to the new conditions produced by the 
lengthening of the incubation period. That the dental laminae possess 
considerable power of adapting their activity to the requirements of 
the jaws is proved by the early production of the uniform teeth in 
the lower jaw as compared with their late appearance in the upper. 
The peculiar relations of the dental laminae to the internal (lin- 
gual) epithelial ingrowths, and the mode of development of both internal 
and external (labio-dental) grooves, are also to be attributed to the 
lengthening of the incubation period. Both were probably originally 
developed as infoldings of epidermis, and their present origin from 
solid ingrowths is due to an alteration in their time of appearance. 
Hence arose also the perplexing relations at present existing between 
the various epidermal ingrowths and the enamel organs of the alter- 
nating teeth. 
The exposed Surface of the Jaws. 
There is yet another feature of interest relating to this most 
specialized of dentitions. I have commented, as others have done, 
1) There must also have been a general retardation affecting the 
anterior teeth of the second dentition in all the dentigerous regions, 
since they now begin their development only slightly earlier in em- 
bryonic life than do the teeth of the third dentition. 
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