156 



THE DISSECTION OF PIG EMBRYOS 



tooth along their basal ends (Fig. 163). The enamel is laid down first 

 as an uncalcified fibrillar layer which later becomes calcified in the form of 

 enamel prisms, one for each ameloblast. The enamel is formed first at 

 the apex of the crown of the tooth and extends downward'toward the root. 

 The enamel cells about the future root of the tooth remain cuboidal or 



Tip of tongue 



Epidermis of lip 



Submaxillary duct ^ 

 Sublingtial duct r^- 



j[L Enamel organ of tooth 

 Denial papilla. 



Meckel's cartilage 

 Bone of mandible 



FIG. 162. Parasagittal section through the mandible and tongue of a 65 mrn. human fetus, 

 showing the relations of the first incisor anlage. X 14. 



Enamel 



T .7 .;.v,*''.iiSM. l M : 



Inner enamel layer J5f*V 



(ameloblasts, 

 Enamel prisms 



Dentine and, 

 dentinal fibers 



Odontoblasts 

 Dental pulp- 



FIG. 163. Section through a portion of the crown of a developing tooth, showing the various 



layers (after Tourneux in Heisler). 



low columnar in form, come into contact with the outer enamel cells, and 

 the two layers constitute the epithelial sheath of the root ; it does not pro- 

 duce enamel prisms (Fig. 164). 



The Dental Papilla The outermost cells of the dental papilla, at 

 the end of the fourth month, arrange themselves as a definite layer of 

 columnar epithelium. Since they produce the dentine, or dental bone, 



