624 Jacob Parsons Schaeffer. 



This condition is, however, of short duration because the defini- 

 tive palate soon begins to form, and the lateral walls of the nasal 

 fossae thus become limited inferiorly. In the formation of the 

 definitive palate the nasal fossae appropriate a considerable por- 

 tion of the primitive oral cavity. This is readily understood 

 when we recall the superior boundary of the primitive oral cavity 

 dorsally. Ventrally the mouth cavity is bounded superiorly by 

 the primitive palate, but dorsally the mouth cavity extends into 

 the future nasal cavity until the formation of the definitive palate 

 establishes a roof for the mouth cavity dorsally (figs. 8, 9, 11, 13, 

 14, and 15). 



The formation of the definitive palate 



A reference to figs. 10, 13, 14, and 15 will indicate the stages 

 in the formation of the definitive palate, and the manner in which 

 the lateral nasal wall becomes limited inferiorly. The first step 

 in the production of the definitive palate is the appearance of the 

 palatal ridges. These are more or less wedge-shaped processes 

 which grow caudally and somewhat medially from the medial 

 sides of the maxillary processes. The palatal processes appear 

 from the forty-fifth to the forty-eighth day of embryonal life, and 

 at first hang almost vertically towards the mouth cavity, on either 

 side of the tongue (fig. 13). They extend from the line of union 

 between the medial nasal and the maxillary processes, where they 

 are continuous with the primitive palate, dorsally to the wall of 

 the pharynx, where they are continuous with the palato-pharyn- 

 geal folds. The palatal processes limit the lateral walls of the 

 cavum nasi inferiorly. 



It will be noticed that the tongue is at first between the palatal 

 processes, i.e., the processes extend below the level of the dorsum 

 of the tongue (figs. 10 and 11). Soon the tongue sinks and comes 

 to occupy a lower position in the mouth cavity. With this change 

 of the tongue the palatal processes become rotated from an almost 

 vertical and sagittal plane to a horizontal plane (compare figs. 

 13 and 14). The processes now shortly meet in the median plane 



