56 THE DEVELOPMENT OF THE SALIVARY GLANDS IN MAN 



identity of this sprout with the mesal flange of the 20 millimeter em- 

 bryo is suggested by its direction, but it springs from a point close to 

 the frenulum, and terminates craniad of the level at which the flange 

 showed signs of proliferation. Either it has been carried bodily for- 

 ward by unec|ual growth along the sulcus, which is improbable, or 

 its advance along the line of its keel has been accompanied by a loss 

 of its caudal portion. 



The lateral hmit of the alveoHngual gutter is given by the 

 alveolar sulcus (Fig. 23, 14), which, wliile preserving the general 

 topography of the 20 millimeter embryo, has altered in extent and 

 relative size. It, extends caudad only to the region of the lingual 

 crossing, where it terminates somewhat abruptly. Craniad it loses 

 depth by the gradual reduction of its mesal wall, and comes to form 

 a mere angle at the lateral border of the alveolingual gutter. Here 

 it is continued in a direction that causes it to pass at first dorsal, and 

 finally lateral, to Meckel's cartilage, becoming indistinguishable a 

 little in advance of the plane of the symphysis. Before it has left 

 its position ental to the cartilage, it is connected by a shallow circumflex 

 furrow with the lingual sulcus, the two sulci defining a shght convexity 

 of the floor of the alveolingual region, the anlage of the plica subun- 

 gualis. Over this and the adjacent segments of the bordering sulci, 

 the epithelium is thickened. Along the mesal portion of the circum- 

 flex sulcus (jp) it forms a keel projecting ventrad and entad into the 

 subjacent mesenchyme, from which arise two small soHd epithelial 

 plugs, the mesal of which is the larger, the anlages of the lesser sub- 

 lingual glands. Owing to the rapid inward sweep of the sulcus, the 

 lateral plug appears in section intermediate between the two sulci ; 

 its attachment to the keel is clear in reconstruction (Fig. 21, 16). 

 Conditions on the two sides are closely similar. The circumflex sul- 

 cus, to which these sprouts of the lesser sublingual glands are attached, 

 appears in the 22 millimeter embryo as a new element. In the descrip- 

 tions of older embryos which follow it is not recognized as distinct 

 from the alveolar sulcus, while the prolongation of the latter ectal to 

 Meckel's cartilage is not mentioned. Apparently the alveolar sulcus 

 loses a primitive cranial extremity, which is replaced by the circum- 

 flex sulcus. It is in connection with this neomorph that the anlages 

 of the lesser sublingual glands appear. A keel prolongs its fundus into 



