STRUCTURE OF THE VERTEBRATES 



141 



tions of this formula, a general reduction in number being typ- 

 ical. The carnivores tend to lose the molars, and the rodents 

 usually have no canines and anterior premolars. The loss of 

 these teeth leaves a wide diastema, or space, between the anterior 

 and posterior teeth. The Cetacea, which have teeth, always have 

 an increased number. 



A. Man 



B. Rabbit 



Fig. 78. Mammal Jaws, showing position of Teeth. Observe the wide 

 space (diastema) between the incisors and premolars of the rabbit. 



Development of Teeth. As one would suspect, the verte- 

 brate teeth and the dermal denticles have may points of devel- 

 opment in common. (See page 109.) The ectoderm covering the 

 body swings over to line the lips, gums and part of the mouth 

 cavity, and it is from the gum ectoderm that the dental vesicles 

 develop. From the embryonic ectoderm a cord of cells pushes 

 downward, the inner end enlarging to form a hollow vesicle. As 

 this ectodermal vesicle meets the knot of mesodermal cells which 

 forms a papilla, the vesicle invaginates to form a double cup. 

 The cells of both mesoderm and ectoderm then begin secreting 

 an inorganic deposit of hard material. The mesodermal body of 

 the tooth is the dentine, the ectodermal cover is the enamel. The 

 latter is the hardest and most completely inorganic substance 

 of the body. As the tooth takes form it is pushed upward un- 

 til it breaks through the gum tissues. The papilla, which con- 

 tains nerves and blood vessels, forms the pulp cavity inside the 

 tooth. The tooth proper has neither nerves nor blood vessels, 

 the pain from tooth decay or a dentist's drill being transmitted 

 to the pulp cavity. 



The majority of mammals have teeth with closed pulp cavi- 

 ties, and these teeth do not grow after eruption is completed un- 

 less the top of the tooth is worn off and the dentine forming 



