1 54 



THE DISSECTION OF PIG EMBRYOS 



flange elongates, and, in embryos of 17 mm., separates from the parent 

 epithelium, forming a tubular structure that opens into the mouth cavity 

 near the front end of the original furrow. The tube grows back into the 

 region of the external ear, branches, and forms the gland in this region, 

 while the stem portion of the tube becomes the parotid du'ct. Acinus 

 cells are present at five months. 



The submaxillary gland arises at 1 1 mm. as an epithelial ridge in the 

 alveolo-lingual (i.e., jaw-tongue) groove, its cephalic end located caudal to 

 the frenulum of the tongue. The caudal end of the ridge soon begins to 

 separate from the epithelium and extends caudad and ventrad into the 

 submaxillary region, where it enlarges and branches to form the gland 

 proper; its cephalic unbranched portion, persisting as the duct, soon hol- 

 lows out. 



The sublingual gland develops in 24 mm. embryos as several solid 

 evaginations of epithelium from the alveolo-lingual groove (Fig. 163). 

 This group, usually regarded as a sublingual gland, really consists of the 

 sublingual proper, with its ductus major and of about ten equivalent 

 alveolo-lingual glands. Mucin cells have appeared by the sixteenth week. 



DEVELOPMENT OF THE TEETH 



The teeth have a double origin. The enamel is from ectoderm, the 

 dentine and cement mesodermal. 



Enamel Organ. There first appears in embryos of about n mm. an 

 ectodermal downgrowth, the dental ridge, or lamina, on the future alveolar 

 portions of the upper and lower jaws (Fig. 1 59) . These laminae are parallel 

 and mesial to the labial grooves. At intervals, on each curved dental 



Lower lip 



Mandible 



' Dental papilla 

 Denial lamina. 

 A B 



FIG. 159. Early stages in the development of the teeth (Rose). A, at 17 mm. (X 90); B, 



at 41 mm. (X 45). 



lamina, a series of thickenings develop, the anlages of the enamel organs, 

 which will form enamel and serve as the molds of the future teeth (Fig. 

 1 60). Soon, the ventral side of each enamel organ becomes concave (40 

 mm. C H) forming an inverted cup, and the concavity is occupied by dense 

 mesenchymal tissue, the dental papilla, or anlage of the dentine and pulp 



