DEVELOPMENT OF THE TONGUE I49 



and this portion of the folds forms the soft palate and the uvula (Fig. 152). The 

 unfused backward prolongations of the palatine folds give rise to the pharyngo- 

 palatine arches, which are taken as the boundary Kne between the oral cavity 

 proper and the pharynx in adult anatomy. 



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, Anat. Hefte, Bd. 36, 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, which shows the 



Nasal septum 



- ^ W I''' S^'' ■ 

 Lateral palatine t'°''''f~''-^^r]rZ^Pz I' -A .■ ■ 'iPl'Prolif crating cells 



^Lateral palatine process 



Tongue- 



-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. 



right palatine fold in a horizontal position, although the left fold projects ventral to the 

 dorsum of the tongue. A region of cellular proliferation may be seen on the under side of 

 each process. 



Anomalies. — The lateral palatine processes occasionally fail to unite in the middle Une, 

 producing a defect known as cleft palate. The extent of the defect varies considerably, in 

 some cases involving only the soft palate, while in other cases both soft and hard palates are 

 cleft. It may also be associated with hare lip. 



DEVELOPMENT OF THE TONGUE 



The development of the tongue 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. 155, I). 

 From the surface, the razor blade should be directed obliquely dorsad 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 (II). Now sever the remaining portion of the head from the 

 body by a transverse section in a plane parallel to the first (III) . Place the ventral portion 

 of the head in a watch glass of alcohol, and, under the dissecting microscope, remove that 

 part of the preparation cranial to the mandibular arches. Looking down upon the floor 

 of the pharynx, remove any portions of the lateral pharyngeal wall which may stfll interfere 

 with a clear view of the pharyngeal arches as seen in Figs. 98 and 156. Permanent mounts 

 of the three stages mentioned above may be made and used for study by the student. 



