204 DEVELOPMENT OF SALIVARY GLANDS IN THE DOMESTIC CAT 



but slight groove with thickened epithelium. Lateral to it the epi- 

 thelium of the floor becomes thinner and an interval gradually forms 

 and widens between it and the thicker epitheUum of the buccal sulcus, 

 which gives rise to the orbital fold, the parotid and orbitoparotid 

 elements. The superior dental ridge is the last to emerge. At 13.5 

 millimeters its epithelial thickening is still continuous with that of 

 the buccal sulcus (Fig. 49, 64). These anlages are lirst defined in the 

 plane of the angulus oris; caudad they diminish and are soon lost. 

 This circumstance would seem to indicate that their separation from 

 the primitive thickening depends upon the broadening of the marginal 

 ca\ity, the epithelium thinning in places as the area to be covered 

 increases in width ; the continuous thickening persisting longest 

 caudad, where the transverse increment is least. 



The lateral border of the mouth, or more particularly of the maxillo- 

 mandibular plane, is the buccal sulcus. This does not long retain the 

 simple curvature present in the 8.5 millimeter embryo. As the mouth 

 lengthens, the sulcus develops a marked sagittal arch ascending from 

 the angulus oris to the orbital angle and thence making a rather abrupt 

 descent to the root of the first entodermal pouch (Figs. g6, 97). This 

 curvature is gradually acquired in embryos of 10-18 milhmeters and 

 persists as long as the furrow retains its continuity. In horizontal 

 projection the sulcus is a concavocon\-ex. Its region of maximum 

 convexity is the orbital angle (Figs. 35, 97, 5). Here it comes into 

 close relation with the buccomasseteric nerve (Fig. 35, 6g), and gives 

 rise to a broader segment of the orbital inclusion (Figs. 35, 7, and 97). 

 The orbital angle coincides with the crown of the sagittal arch. The 

 sulcus is thence continued craniad in a nearly sagittal direction to 

 within a short distance of the angulus oris, where it turns rather 

 abruptly laterad. In early stages this postangular segment is short 

 and distinctly oblique, becoming more nearly transverse as develop- 

 ment proceeds and actually so in embryos of 20 milhmeters and over. 

 The sagittal segment is the largest portion of the sulcus ; its junction 

 with the postangular portion may conveniently be termed the post- 

 angular bend (Figs. 35, 97, 4). It evidently corresponds to that 

 change in direction of the buccal sulcus at which His found the parotid 

 anlage in the embryo Zw. 



The manner in which the buccal sulcus acquires this course is a 



