202 DEVELOPMENT OF SALIVARY GLANDS IN THE DOMESTIC CAT 



begun to undergo constriction at the line of its union with the oral 

 epithelium (Figs. 14 and 17). In these sections the roof of the marginal 

 ca\-ity presents a small convexity just mesal to the orbital fold. 

 With this the ectopalatine sulcus is continuous. After the separation 

 of the orbital inclusion, this convexity forms the fundus of the secon- 

 dary buccal sulcus, which now constitutes the lateral border of the mouth 

 (vide infra, page 217). Up to the stage of 13.5 milUmeters (Fig. 35) 

 the ectopalatine sulcus retains its continuity with the secondary 

 buccal sulcus, near the end of the orbital inclusion. Later the con- 

 tinuity is lost by the obhteration of the caudal extremity of the furrow 

 (Fig. 97)- 



The palatomandibular plane thus bounded extends as a sagittal 

 strip of nearly uniform breadth from the pharynx to the plane of the 

 anguU oris, there to turn mesad, narrow, and rapidly lose its relief. 

 Throughout it has a dorsolateral inclination, which is progressively 

 diminished craniad as the vertical extent of the palate process is 

 reduced (Figs. 9, 15). The epithehum of its floor gives rise to the 

 inferior dental anlage. Apart from this, the plane retains the character 

 of a dorsolaterally inclined and almost featureless space, until the 

 closure of the palate, when its dorsomedian wall becomes the roof of 

 the ca\aim oris and its ventrolateral is added to the alveolingual 

 region. The caudal portion of the marginal cavity where it is com- 

 posed of this plane alone has an equally uneventful history. It 

 corresponds to the postdental region of the adult mouth, where the 

 vestibule communicates with the ca\'um oris and the whole ca\-ity 

 narrows to the fauces. Here the masseter and internal pterygoid 

 abut upon the mucous membrane, a condition estabhshed for the 

 latter muscle in the embryo of 13.5 millimeters (Fig. 37, yj), a contact 

 which no doubt operates to impede the growth of the maxillomandib- 

 ular plane in this region. 



The ma.xillomandibular plane, bounded by the buccal and ecto- 

 palatine sulci, is triangular, diminishing caudad as the sulci converge. 

 Here it enters into relations with the orbital inclusion, which must 

 subsequently be considered in some detail. The 10.5 millimeter 

 embryo (Figs. 9, 10) may serve to illustrate this condition. In the 

 plane of the'mandibular nerves the marginal cavity lateral to the palate 

 process is reduced to a bhnd fold or flange, the orbital inclusion 



