2o8 DEVELOPMENT OF SALIVARY GLANDS IN THE DOMESTIC CAT 



and the surface situation of the angulus, it is seen that we are dealing 

 with httle more than a point of fusion, and here if anywhere the forma- 

 tion of keels or plates should be minimal. 



In the 13.5 miUimeter embryo (Fig. 35) the buccal sulcus has attained 

 a form which it retains until the transformation of the maxillomandibu- 

 lar plane into the vestibule. It comprises a short postangular segment, 

 a postangular bend (4), a longer more sagittal section extending to 

 the orbital angle, where the direction again becomes more nearly 

 transverse. This oblique terminal segment carries the sulcus close 

 to the first entodermal pouch {84). From the orbital angle caudad 

 the maxillomandibular plane is reduced to a fold, the orbital inclu- 

 sion (7, 8). This it may be recalled is horizontal at the orbital 

 angle (Fig. 38). At 10 millimeters the plan* is horizontal at the angulus, 

 at Ti only the extremity of the postangular sulcus is ventrally inclined. 

 In embryos of this length the orbital inclusion extends beyond the 

 postangular bend, within a very few micra of the angulus. It would 

 seem possible, therefore, that the orbital angle corresponds to the post- 

 angular bend of these embryos, which, becoming involved in the orbital 

 fold, retains the primitive curvature of the stage at which the fold is 

 formed, while in more cranial portions of the sulcus the successive 

 positions of the postangular bend leave no permanent record of their 

 position, the latter increments falling in line with the sagittal segment 

 of the sulcus by a shght degree of remodeb'ng ; or it may prove to 

 be the case that the whole postangular segment is a line where fusion 

 occurs, and that it is the postangular bend and not the angulus oris 

 which traces out the buccal sulcus as it advances. The solution of 

 this point requires a more careful study of the distribution of rudi- 

 mentary keels about the angulus than has yet been made. 



To recapitulate, the oral cavity is lengthened by additions at its 

 cranial end. The angulus oris advances along a line determined by 

 the relation of the maxillary and mandibular processes. As the 

 ma.xillary overhangs the mandibular process laterad, and increasingly 

 towards its tip, the maxillomandibular plane acquires a torsion about 

 a sagittal axis passing from horizontal at the orbital angle to a marked 

 ventrolateral slope craniad. Changes in the internal conformation 

 of the cavity are initiated caudad, and appear due, first, to the con- 

 fining influence of the mandibular nerves, and second, to that of the 



