162 



THE DISSECTION OF PIG EMBRYOS FOR STUDY 



157). Of the alveolo-lingual glands nine or ten may develop on either side in embryos of 40 

 mm. (McMurrich in Keibel and Mall, vol. 2, p. 348-349.) 



The branched anlages of the salivary glands are at first solid and hollow out peripherally. 

 The glands continue growing and enlarging until after birth. Mucin cells may be distinguished 

 by the sixteenth week and acinus cells in the parotid glands at five months (McMurrich). 



THE DEVELOPMENT OF THE TEETH 

 The development of the teeth is described in all the standard textbooks of 

 histology and only a brief account of their origin and structure will be given here. 

 The enamel organs, which give rise to the enamel of the teeth and are the moulds, 

 so to speak, of the future teeth, are of ectodermal origin. There first appears in 

 embryos of 10 to 12 mm. an ectodermal downgrowth, the dental ridge or lamina 

 on the future alveolar portions of the upper and lower jaws (Fig. 154). These 



LF. 



Fig. 154. — Early stages in the development of the teeth. A, 17 mm.; B, 41 mm. (Rose). LF., LFL., 

 labial groove; Pp., dental papilla; UK., lower jaw; uL., lower lip; ZL., dental ridge. 



lamina; parallel and are mesial to the labial grooves, being directed obliquely 

 toward the tongue. At intervals, on each curved dental ridge or lamina a series 

 of thickenings develop, the anlages of the enamel organs (Fig. 155). Soon the 

 ventral side of each enamel organ becomes concave (embryos of 40 mm.) forming 

 an inverted cup and the concavity is occupied by dense mesenchymal tissue, the 

 dental papilla (Figs. 154 B and 156). An enamel organ with dental papilla 

 forms the anlage of each decidual or milk tooth. Ten such anlages are present in 

 the upper jaw and ten in the lower jaw of a 40 mm. embryo. The connection of 

 the dental anlages with the dental ridge is eventually lost. The position of the 

 tooth anlage between the tongue and lip is shown in Fig. 157. 



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

 milk teeth, are developed in the same way along the free edge of the dental 

 lamina median to the decidual teeth. In addition, the anlages of three perma- 

 nent molars are developed on each side, both above and below from a backward or 



