66 



EMBRYOLOGY 



Lateral tongue Thyroid 

 elevations diverticulum 



complete clefts, the gill-clefts, opening from the pharynx on to the exterior; 

 ration, however, does not occur in birds or mammals. The grooves separate a 

 series of rounded bars or arches, the branchial or visceral arches, in which thickening 

 of the mesoderm takes place (Figs. 40 and 41). The dorsal ends of these arches 

 are attached to the sides of the head, while the ventral extremities ultimately 

 meet in the middle line of the neck. In all, six arches make their appearance, 

 but of these only the first four are visible externally. The first arch is named the 



mandibular, and the second the hyoid; the 

 others have no distinctive names. In each 

 arch a cartilaginous bar, consisting of right 

 and left halves, is developed, and with each 

 of these there is one of the primitive aortic 

 arches. 



The mandibular arch lies between the first 

 branchial groove and the stomodeum; from it 

 are developed the lower lip, the mandible, 

 the muscles of mastication, and the anterior 

 part of the tongue. Its cartilaginous bar is 

 formed by what are known as Meckel's carti- 

 lages (right and left) (Fig. 43) ; above this the 

 incus is developed. The dorsal end of each 

 cartilage is connected \vith the ear-capsule 

 and is ossified to form the malleus; the ventral ends meet each other in the region 

 of the symphysis menti, and are usually regarded as undergoing ossification to form, 

 that portion of the mandible which contains the incisor teeth. The intervening 

 part of the cartilage disappears; the portion immediately adjacent to the malleus is 

 replaced by fibrous membrane, which constitutes the spheno-mandibular ligament, 



Entrance to 

 larynx 



Fia. 42. Floor of pharynx of embryo shown in 

 Fig. 40. 



Malleus 



Incus , 



,--- Tympanic ring 

 _'"' Mandible 



{?" Meckel's cartilage 



Hyoid bone 



FIG. 43. Head and neck of a human embryo eighteen weeks old, with Meckel'a cartilage and hyoid bar exposed. 



(After Kolliker.) 



while from the connective tissue covering the remainder of the cartilage the greater 

 part of the mandible is ossified. From the dorsal ends of the mandibular arch a 

 triangular process, the maxillary process, grows forward on either side and forms 

 the cheek and lateral part of the upper lip. The second or hyoid arch assists in 

 forming the side and front of the neck. From its cartilage are developed the styloid 

 process, stylohyoid ligament, and lesser cornu of the hyoid bone. The stages prob- 



