158 THE DISSECTION OF PIG EMBRYOS FOR STUDY 



embryo which shows the right palatine fold in a horizontal position, although the 

 left fold projects ventral to the dorsum of the tongue. A region of cellular pro- 

 liferation may be seen on the under side of each process. 



At the end of the second month the palatine bones begin to develop in 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. The un- 



sNasal septum 



Lat palatine . 

 process 



Tongu 



Fig. 149. — 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. 



fused backward prolongations of the palatine folds give rise to the arcus pharyn- 

 go-palatini, which is taken as the boundary line between the oral cavity proper 

 and the pharynx in adult anatomy. 



The lateral palatine processes occasionally fail to unite in the middle line, producing a 

 defect known as cleft palate. The extent of the defect varies considerably, in some cases involv- 

 ing only the soft palate, while in other cases both soft and hard palates are cleft. 



THE DEVELOPMENT OF THE TONGUE 

 The tongue develops as two distinct portions, the body and the root, separated 

 from each other by a V-shaped groove, the sulcus terminalis. Its development 

 may be studied from dissections of pig embryos 6, 9 and 13 mm. long. As the 

 pharynx is bent nearly at right angles, it is necessary to cut away its roof by two 

 pairs of sections passing in different planes. The first plane of section cuts 

 through the eye and first two branchial arches just above the cervical sinus (Fig. 

 1 50, I) . From the surface, the razor blade should be directed obliquely dorsal 

 in cutting toward the median line. Cuts in this plane should be made from either 

 side. In the same way make sections on each side in a plane forming an obtuse 

 angle with the first section and passing dorsal to the cervical sinus (Fig. 150, II). 

 Now sever the remaining portion of the head from the body by a transverse sec- 



