DEVELOPMENT OF THE TONGUE 151 



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 

 line, 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 p'assing 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 to ward 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, re- 

 move 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 still interfere with a ^ I55 ._ Lateral view of the 



clear view of the pharyngeal arches, as seen in head of a ? mm pig embryo The 

 Figs. 98 and 156. Permanent mounts of the three three lines ind i ca te the planes of sec- 

 stages mentioned above may be made and used tions to be made in dissecting the 

 for study by the student. tongue as described in the text. 



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

 separated from each other by a V-shaped groove, the sulcus terminalis. In 

 both human and pig embryos the body of the tongue is represented by three 

 anlages that appear in front of the second branchial arches. These are 

 the median, somewhat triangular tuberculum impar, and the paired lateral 

 swellings of the first, or mandibular arches, all of which are present in 

 human embryos of 5 mm. (Figs. 98 and 157 A}. At this stage, a median 

 ventral elevation formed by the union of the second branchial arches (and, 

 according to some workers, the third as well) constitutes the copula. This, 

 with the portions of the second arches lateral to it, forms later the base, 

 or root, of the tongue. Between it and the tuberculum impar is the point 

 of evagination of the thyreoid gland. The copula also connects the tuber- 

 culum impar with a rounded prominence that is developed in the mid- ven- 

 tral line from the bases of the third and fourth branchial arches. This is 

 the anlage of the epiglottis. 



