1102 



SPLANCHNOLOGY 





mandibular arch, and laterally by the maxillary processes (Fig. 978) . With the 

 inward growth and fusion of the palatine processes (Figs. 50, 51), the stomodeum 

 is divided into an upper nasal, and a lower buccal part. Along the free margins 

 of the processes bounding the mouth cavity a shallow groove appears; this is 

 termed the primary labial groove, and from the bottom of it a downgrowth of 

 ectoderm takes place into the underlying mesoderm. The central cells of the 

 ectodermal downgrowth degenerate and a secondary labial groove is formed; by 

 the deepening of this, the lips and cheeks are separated from the alveolar processes 

 of the maxillae and mandible. 



The Salivary Glands. The salivary glands arise as buds from the epithelial 

 lining of the mouth; the parotid appears during the fourth week in the angle 



between the maxillary process and the 

 mandibular arch; the submaxillary ap- 

 pears in the sixth week, and the sublin- 

 gual during the ninth week in the hollow 

 between the tongue and the mandibular 

 arch. 



The Tongue (Figs. 979 to 981). The 

 tongue is developed in the floor of the 

 pharynx, and consists of an anterior or 

 buccal and a posterior or pharyngeal part 

 which are separated in the adult by the 

 V-shaped sulcus terminalis. During the 

 third week there appears, immediately 

 behind the ventral ends of the two halves 

 of the mandibular arch, a rounded 

 swelling named the tuberculum impar, 

 which was described by His as un- 

 dergoing enlargement to form the 

 buccal part of the tongue. More re- 

 cent researches, however, show that 

 this part of the tongue is mainly, if 

 not entirely, developed from a pair 

 of lateral swellings w r hich rise from 

 the inner surface of the mandibular arch and meet in the middle line. The tuber- 

 culum impar is said to form the central part of the tongue immediately in front 



Future apex of nose 

 Medial nasal process 



Olfactory pit 

 Lateral nasal process 

 Globular process 

 Maxillary process 

 Stomodeum 



Mandibular arch 



FIG. 978. Head end of human embryo of about thirty 

 to thirty-one days. (From model by Peters.) 



Lateral tongue Thyroid 



diverticulum 



Lateral tongue swellings 



Entrance to 

 larynx 



^mmmmmtmm^mmmm^i^mr 



FIG. 979. Floor of pharynx of human embryo 

 >out twenty-six days old. (From model by 



about 

 Peters.) 



Entrance to 

 larynx 



Arytenoid 

 swellings 



FIG. 980. Floor of pharynx of human embryo 

 of about the end of the fourth week. (From 

 model by Peters.) 



of the foramen cecum, but Hammar insists that it is purely a transitory structure 

 and forms no part of the adult tongue. From the ventral ends of the fourth arch 



