158 



THE DISSECTION OF PIG EMBRYOS 



the tooth gives rise to a dense outer layer and a more open inner layer of 

 fibrous connective tissue. These layers form the dental sac (Fig. 164). 

 Over the root of the tooth a layer of osteoblasts, or bone forming cells, de- 

 velops, and, the epithelial sheath formed by the enamel layers having 

 disintegrated, these osteoblasts deposit about the dentine a layer of spe- 

 cialized bone, known as the cement. The cement layer contains typical 

 bone cells but no Haversian systems. As the tooth grows and fills the 

 alveolus, the dental sac becomes a thin, vascular layer, the peridental 

 membrane. This has fibrous attachments to both the alveolar bone and 

 the cement of the tooth. 



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

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



Labial 

 roove 



Dental 

 lamina 



FIG. 165. Dental lamina and anlages of the upper milk teeth in a 



(Rose). 



115 mm. human fetus 



sixth to eighth month. 

 eighth to twelfth month. 



approach the surface and break through the gums. The periods of 

 eruption of the various milk, or decidual teeth vary with race, climate, 

 and nutritive conditions. Usually these teeth are cut in the following 

 sequence : 



Median Incisors 



Lateral Incisors 



First Molars .................. ......... ... twelfth to sixteenth month. 



Canines .................................. seventeenth to twentieth month. 



Second Molars ........................... twentieth to thirty-sixth month. 



The permanent teeth develop precisely like the temporary set. The 

 anlages of those permanent teeth which correspond to the decidual, or 

 milk teeth, are developed in another series along the free edge of the den- 

 tal lamina (Fig. 160 D) and come to lie mesad of the decidual teeth (Fig. 

 1 66). In addition, the anlages of three permanent molars are developed 

 on each side, both above and below, from a backward or aboral extension 



