DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



149 



to the major part of the cheek and the lateral portion of the upper lip. The 

 fusion between the maxillary and nasal processes, as seen on surface view, is 

 coincident with and a part of J;he separation of the nasal cavity from the oral 

 cavity (see page 319). The nose itself is at first a broad, flat structure, but 

 later becomes elevated above the surface of the face, with an elongation and a 

 narrowing of the bridge. 



Mid-brain 



Cerebral hemisphere 



Lat. nasal process 

 Nasal pit 



Med. nasal process 

 Angle of mouth 



Eye 



Naso-optic furrow 



Maxillary process 



B Mandibular process 



Branchial grooves 



^B Branchial arch II 



FIG. 135. Ventral view of head of 113 mm. human embryo. Rabl. 



The lower jaw, lower lip and chin are formed by the mandibular processes of 

 the first branchial arch (Figs. 134, 136, 137). At first the chin region is rela- 

 tively short, but broad in a transverse direction. Later it becomes longer and a 

 transverse furrow divides the middle portion into lower lip and chin (Fig. 137). 



The Extremities. 



The limb buds appear in human embryos about the end of the third week as 

 small, rounded protuberances on the ventro-lateral surface of the body. The 

 upper limb buds arise just caudal to the level of the cervical flexure, the lower 

 opposite the sacral flexure (Figs. 123, 124). The upper appear first, the lower 

 following shortly, and the difference in time in the appearance of the upper 

 and lower buds is followed by a difference in degree of development, the 

 upper extremities maintaining throughout f cetal life a slight advance in develop- 

 ment over the lower. 



