DEVELOPMENT OF SALIVARY GLANDS IN THE DOMESTIC CAT 207 



along tliis Kne would describe an arch with its convexity dorsad, the 

 buccal sulcus ; the maxillomandibular plane would gain a lengthen- 

 ing cranial segment of ventrolateral inchnation, the degree of which 

 would also increase craniad ; and the regions of horizontahty, marking 

 t)ie site of the angulus of the lo millimeter embryo, would become 

 relatively more and more caudal in situation. The sections of the 

 i,VS millimeter embryo indicate the fulhliment of these conditions. 

 In Fig. 36 the section passes through the parotids ; its plane is given 

 by the position of these anlages in the model (Fig. 35). Here the 

 maxillomandibular plane has a slight ventrolateral inclination. 

 Only so far caudad as the region of the masseter (Figs. 37, 38) is the 

 horizontal region of the plane encountered, here undergoing compres- 

 sion in its lateral portion to form the orbital fold. The arch of the 

 buccal sulcus is shown by its successively higher positions in Figs. 36- 

 38. In Fig. 39 its descent is begun. All of these conditions, in this 

 and the preceding embryos, are satisfactorily explained by the assump- 

 tion of an active advance of the angulus by progressive fusion of the 

 opposing epithelia of the maxillary and mandibular processes, thus 

 incorporating into the oral cavity preangular portions of the interval 

 between the processes, these retaining the inchnation ventrolaterad 

 that was impressed upon them while yet preangular in position. 



When we turn from such considerations as these to look for direct 

 evidences of fusion at the angulus, it must be admitted that they are 

 inconspicuous. The postangular portion of the buccal sulcus fre- 

 quently has a minute keel projecting from its fundus (Fig. 24), rarely 

 the surface furrow has the Uke. Platelike formations, as in Figs. 3 

 and 9, while constant, have never a considerable extent. Meager as 

 they are, these appearances are more readily explained as the result 

 of fusion than otherwise, nor can we argue from their niinuteness 

 that the degree of fusion is itself inconsiderable. Fusion between 

 processes must mean not only blending of their surface epithelia, but 

 also reduction of the plates so formed. The degree of plate forma- 

 tion observable at any time is not an index of the fusion per se, but 

 of the degree by which the reduction lags behind it. If the two pro- 

 cesses, fusion of epitheHum and destruction of epithelium, advance 

 by equal steps, no gross evidences of the process could be expected. 

 From what has been said of the direction of the postangular sulcus 



