THE DISSECTION OF PIG EMBRYOS 



the incisive fossa, and laterad between the two processes openings persist for 

 some time, which are known as the incisive canals (of Stenson). 



In human embryos these changes are essentially identical (Fig. 153). 

 The lateral palatine processes begin to fuse cranio-caudally at about the 

 end^of.the second month. At the same time, palatine bones first appear in 



Median palatine process 



Processus 'globularis 

 ' (Median nasal process) 



Lateral palatine process 



Maxillary process 



FI G - X 53- The roof of the mouth of a human embryo about ten weeks old, showing the 

 development of the palate (after His). X 9. In the roof of the mouth are the openings of 

 the primitive choanae. 



the lateral palatine folds and thus form the hard palate. Caudally, the 

 bones do not develop, and this portion of the folds forms the soft palate and 

 the uvula (Fig. 152). The unfused backward prolongations of the pala- 

 tine folds give rise to the pharyngo-palatine arches, which are taken in 

 adult anatomy as the boundary line between the oral cavity proper and 

 the pharynx. 



Nasal septum 



\ 



Proliferating cells 

 Lateral palatine process 



Mandible 



FIG. 154. Section through the jaws of a 25 mm. pig embryo, to show the change in the position 

 of the palatine processes with reference to the tongue. 



After the withdrawal of the tongue, the lateral palatine processes take up a horizontal 

 position and their edges are approximated because the cells on the ventral sides of the folds 

 proliferate more rapidly than those of the dorsal side (Schorr, 1908). That the change in 

 position of the palatine folds is not mechanical, but due to unequal growth, may be seen 

 in Fig. 154, a section through the palatine folds of a pig embryo that shows the right 



