200 



TEXT-BOOK OF EMBRYOLOGY. 



The incisive bone is probably not derived from the maxillary process of the first visceral 

 arch, but from the fronto-nasal process. The question thus arises as to whether it is derived 

 from both the middle and lateral nasal processes or only from the middle. According to 

 Kolliker's view, the lateral nasal process takes no part in the formation of the incisive bone. 

 It is derived from the middle process, hence genetically it is a single bone on each side. 

 According to Albrecht's view the incisive bone is genetically composed of two parts, one 

 derived from the lateral, the other from the middle nasal process. While the matter is not 

 one of great importance merely from the standpoint of development, it has an important 

 bearing on the question of certain congenital malformations, e.g., hare lip, and will be 

 discussed further under that head (p. 216). 



In the mandibular process of the first visceral arch, the mandible develops as 

 a bone which is partly of intramembranous and partly of intracartilaginous 

 origin. In the first place a rod of cartilage, known as Meckel's cartilage, 

 forms the core of the mandibular process and extends from the distal end of the 

 process to the temporal region of the skull, where it passes between the tympanic 



Medial line 

 Canine alveolus 



Molar alveolus 



Palate bone 

 (horizontal part) 



FIG. 179. Ventral aspect of hard palate of human embryo of 80 mm. Kollmann's Atlas. 



bone and the periotic capsule and ends in the tympanic cavity of the ear (Fig. 

 174). During the sixth week of fcetal life, intramembranous bone begins to 

 develop in the mandibular process. In the region of the body of the mandible 

 the bone encloses the cartilage, but in the region of the ramus and coronoid 

 process the cartilage lies to the inner side of the bone. Development is further 

 complicated by the appearance of cartilage in the region of the middle incisor 

 teeth and on the coronoid and condyloid processes. These pieces of cartilage 

 form independently of Meckel's cartilage and subsequently are replaced by the 

 bone which constitutes the corresponding parts of the mandible. The part of 

 Meckel's cartilage enclosed in the bone disappears; the part to the inner side of 

 the ramus is transformed into the sphenomandibular ligament. (See Fig. 180.) 

 In each half of the second branchial arch a rod of cartilage develops, which 

 extends from the ventro-medial line to the region of the periotic capsule. The 



