TERATOGENESIS. 609 



results in abnormally small lower jaws micrognaihy, or in almost complete 

 absence of the lower jaws agnathus; in the latter case the ears are brought 

 together in the ventro-medial line synotus. Rarely the mandible is partly 

 duplicated, due to the development of a secondary mandibular process 

 dignathus. 



Clefts in the upper lip, maxilla and palate follow the lines of primary union 

 of the processes which form these structures (consult Figs. 136 and 137). The 

 cleft may affect the lip alone, may be single or double, but is always lateral 

 hare lip (cheiloschisis). It may affect the lip and maxilla (cheilognathoschisis) , 

 or the lip, maxilla and palate (hare lip and cleft palate, cheilognathouranos- 

 chisis). (For a further discussion of hare lip and cleft palate, see p. 212). 



Occasionally there is an entire lack of union between the naso-frontal process 

 and the maxillary process. The result is an oblique cleft which extends up- 

 ward from the mouth oblique facial cleft (cheilognathoprosoposchisis) . The 

 processes which form the boundaries of the mouth slit (maxillary and mandib- 

 ular processes) sometimes fail to fuse to the normal extent, thus giving rise to 

 macrostomus; or the fusion may proceed beyond the normal limit, giving rise to 

 microstomus; rarely complete fusion of the processes on one side with each other 

 and with their fellows of the opposite side results in closure of the mouth slit 

 astomus or atresia oris. Clefts in the lower lip, due to imperfect union of the 

 two mandibular processes in the medial line, are rare. 



The branchial arches (apart from the first which has already been con- 

 sidered) and the branchial grooves are also subject to defective developmental 

 processes. Malformations of the ear, with closure of the external auditory 

 meatus, due to abnormal development of the first groove and surrounding parts, 

 are sometimes met with either alone or in connection with other facial defects. 

 Cervical fistula are the results of imperfect closure of some of the grooves along 

 with rupture of the membranes that separate the bottoms of the external 

 grooves from the bottoms of the internal grooves or pharyngeal pouches. The 

 fistula may be complete, that is, there may be a communication between the 

 pharyngeal cavity and the exterior; or it may be incomplete, opening either 

 into the pharynx, or on the surface of the body. The internal opening of a 

 cervical fistula is usually in the lower part of the pharynx or in the posterior 

 palatine arch near the tonsil. The external opening varies in position. It is 

 usually situated near the sterno-clavicular articulation, or at the inner or 

 outer edge of the sterno-mastoid muscle. The majority of cervical fistulae are 

 probably derived from the second branchial groove. They all have the form of 

 narrow canals lined with mucous membrane. Medial cervical fistulae, the ex- 

 ternal openings of which are situated in the medial line, are rare. 



It sometimes happens that during the closure of the branchial grooves por- 

 tions of the walls of the grooves becomes enclosed within the walls of the 



