i66 



THE DISSECTION OF PIG EMBRYOS FOR STUDY 



teeth grow and fill the alveoli the dental sac becomes a thin vascular layer con- 

 tinuous externally with the alveolar periosteum, internally with the periosteum of 

 the cement layer of the tooth. 



Dental sac 



Inner layer 



Outer enamel cells 



Enamel pulp 



Enamel 



> Epithelial sheath 



I J 

 I 



Dentine 



Odontoblasts 



Dental papilla (future pulp) 



Blood-vessel 



Bony trabecula of the lower 

 jaw 



FIG. 159. Longitudinal section of a deciduous tooth of a newborn dog. X 42. The white 

 spaces between the inner enamel cells and the enamel are artificial, and due to shrinkage (Lewis 



and Stqhr). 



^ s ^- - 



When the crown of the tooth is fully developed the enamel organ disinte- 

 grates, and as the roots of the teeth continue to grow their crowns approach the 

 surface and break through the gums. The periods of eruption of the various 



