DEVELOPMENT OF SALIVARY GLANDS IN THE DOMESTIC CAT 283 



cesses. An opposition of forces thus engendered produces beside the 

 tongue a sulcus, and where the additional factor of compression by 

 the submaxillary ganglion is present, a flange. But the flange far 

 exceeds in depth the open sulcus which continues it craniad. While 

 this may be assigned solely to the more active and earlier established 

 growth of its layers, there are certain facts which render probable 

 that it is assisted by a greater degree of inrolling of epithelium from 

 the tongue and alveolingual gutter. The tongue as it is followed 

 caudad diminishes in its elevation above the level of the gutters, while 

 they, too, narrow as the region of the flange is approached. The latter 

 is plainly apparent to one running over the sections, but from the 

 absence of an alveolar sulcus in the cat it is less conspicuously evident 

 than in the human embryo (Part II, Figs. 4, 6-19). Indirect evidence 

 to the same effect is afforded by the adult condition of the Rivinian 

 glands in both forms. These develop lateral to the submaxillary at 

 first, and subsequently the caudal members of the series become dorsal 

 to the submaxillary line and continuous with the isthmian elements 

 (Weber's gland). The most plausible explanation seems to be a 

 shift mesad of primitively lateral parts, which is precisely what the 

 inrolling of alveolingual surface epithelium into the submaxillary 

 flange would entail. In this displacement, areas of surface epithelium 

 primitively close to the sulcus would occupy ventral (aboral) positions 

 in the flange, while more lateral elements would be dorsal. 



The anlage estabhshed, we have now to consider its cranial advance, 

 the separation of its duct, and the extension distad of its sprout. 



The advance has been seen to depend in part upon the continuation 

 of the flange beyond the lingual crossing, in part upon the presence of 

 the primitive keel. The transition is very gradual between these 

 two regions of the anlage, but is always prelingual. In older embryos 

 the nerve crosses in two divisions ; the transition is usually nearer 

 the smaller cranial branch in the period of 15-17 miUimeters. The 

 flange diminishes, giving place to an open sulcus with a fundal keel; 

 this becomes parietal and finally stops altogether. The conditions 

 topical of an anlage of this period are shown in the model of a 16 milli- 

 meter embryo (Fig. 140). A slight tendency of the keel to become 

 parietal in the early anlage has been referred to. In the region of the 

 intermediate segment of the lingual sulcus tliis becomes pronounced, 



