280 HISTORY OF THE HUMAN BODY 



embryological record, also, with a few exceptions, which may 

 be explained in other ways, is in accord with it. 



A second theory to account for the complex form of molars 

 is that they are in reality, as they are often termed, " double 

 teeth/' and arise from the fusion of several primary germs. 

 This concrescence theory is much older than the first, and had 

 been generally abandoned in favor of the other when its 

 probability in the case of certain forms was recently reasserted, 

 owing to the testimony of embryology. It is thus possible 

 that both theories may be true as applied to different cases, 

 the tritubercular method being the more general. 



A widespread phenomenon among mammals is that of the 

 replacement at a definite period of certain of the anterior 

 teeth by a second set, and in respect to this power mammals 

 are classed as diphyodont, in which such a replacement oc- 

 curs, and monophyodont, where but one set appears. Al- 

 though in man and in many domestic animals, in which this 

 procedure was first studied, the distinction between the two 

 sets is clear and definite, such is. not the case among certain 

 of the mammals, and careful embryological records which 

 show numerous cases of rudimentary tooth germs both pre- 

 ceding and succeeding definite teeth, has caused a revision 

 of the entire subject. The matter becomes more compre- 

 hensible by referring to the lower vertebrate Classes, espe- 

 cially reptiles, where the papilla of each physiologically active 

 tooth is associated with a succession of additional tooth germs 

 in different stages of maturity, and designed to replace the 

 functional tooth in case of injury (Fig. 77). 



In a vitally important tooth, as in the poison fangs of ser- 

 pents, at least one replacement tooth, nearly ready for use, 

 lies continually by the side of the functional one, and may 

 develop almost at a moment's notice in case of the loss of the 

 latter. This power, limited to a single generation of replace- 

 ment teeth, restricted to that part of the jaws anterior to the 

 true molars, and arranged to assert itself at a certain definite 

 period in the case of each tooth would result in the two den- 

 titions of the typical diphyodont Mammalia (Fig. 78, A), and 



