282 de\t;lopment of saliv.-vry gl.\xds in the domestic cat 



the preformed keel would be found at its fundus. This element, how- 

 ever, ceases to be distinguishable from the flange as soon as the latter 

 is formed. It is only an inference that it retains its morphogenic 

 individuahty, and, constituting the free border of the flange, is the 

 actual site of proliferation which gives rise to the subma.xillary duct. 

 The sections of the 13.5 millimeter embryo. No. 78 (Figs. 126-127), 

 show this early proliferation rather on the lateral aspect of the flange, 

 and suggest that even here there is a slight tendency- to shifting of the 

 fundus as the sulcus deepens and consequent displacement of the keel 

 to a parietal position, such as occurs to an increasing degree craniad. 

 We have now to enter upon the question of the mode of formation 

 of the flange, and at the outset it must be borne in mind that there is 

 here no reduction by compression of an open sulcus to produce a 

 secondary flange, as in the case of the parotid and orbital inclusion. 

 Here the flange is primary. That it is the result of proliferation un- 

 compUcated by folding is a supposition little in harmony %\ath its 

 structure. The fissure that indents its base is too deep and constant 

 to be regarded as a mere concomitant of rapid proliferation, and is, 

 further, always continuous with the periderm axis. It is difficult, also, 

 to interpret this periderm as developed in situ secondarily, for it is 

 present from the inception of the flange. Above all. the gradual re- 

 duction of the flange craniad is accompanied by a deepening of the 

 fissure, the waUs of which are continuous with the layers of the flange. 

 Granted that the anlage is a true flange, it is yet certain that it is not 

 produced by apposition of the walls of a primarily open sulcus. There 

 are here two alternatives, accelerated growth in the epitheUum im- 

 mediately adjacent to the as yet angular lingual furrow on both sides, 

 or a process of inroUing of the epithelium of the tongue and alveolingual 

 region towards their common boundary, the lingual sulcus. An ante- 

 cedent necessity in either case is the presence of factors tending to 

 prevent the free expansion of the region, and so leading to the produc- 

 tion of a sulcus or fold. This condition is satisfied in the relation of 

 the tongue to the palate processes and in particular to the roof of the 

 mouth between them. Here, as His has pointed out, the growth of 

 the tongue is resisted, and the organ as a whole tends to sink ventrad 

 between the alveolingual gutters as development proceeds. They 

 in turn are limited laterad by the enlarging region of the alveolar pro- 



