160 THE DISSECTION OF PIG EMBRYOS 



The teeth of vertebrates are homologues of the placoid scales of elasmobranch fishes 

 (sharks and skates). The teeth of the shark resemble enlarged scales, and many genera- 

 tions of teeth are produced in the adult fish. In some mammalian embryos three, or even 

 four, dentitions are present. The primitive teeth of mammals are of the canine type, and 

 from this conical tooth the incisors and molars have arisen. Just how the cusped tooth 

 differentiated whether by the fusion of originally separate units, or by the development 

 of cusps on a single primitive tooth is debated. 



Anomalies. Dental anomalies are frequent and may consist in the congenital absence 

 of some or all of the teeth, or in the production of more than the normal number. Defect- 

 ive teeth are frequently associated with hare lip. Cases have been noted in which, owing 

 to defect of the enamel organ, the enamel was entirely wanting. Third dentitions have been 

 recorded, and occasionally fourth molars may be developed behind the wisdom teeth. 



