154 



TEXT-BOOK OF EMBRYOLOGY. 



During the fourth week the limb buds become elongated, and each bud 

 becomes divided by a transverse constriction into a proximal and a distal por- 

 tion (Figs. 124, 125). The proximal portion remains cylindrical, while the 



Nasa fos ..■ 



Naso-optic furrow — 

 Mouth slit — j 



Branchial groove I ' 



\ 



11 



J" «* 



£ 



v 



I 



ICerebral hemisphere 



■Naso-frontal process 

 ■Lateral nasal process 



■Medial nasal process 

 N Maxillary process 



■Mandibular process 



Fig. 136. — Ventral view of head of 13.7 mm. human embryo. His. 



distal portion becomes somewhat broader and considerably flattened. Dur- 

 ing the fifth week the digits appear (see below). During the sixth week the 

 proximal portion of each bud is subdivided by a transverse constriction into 

 two segments (Fig. 127). Thus each extremity as a whole is divided into three 



Branchial groove I" 

 (external ear) 



Nose 



l> 





Lat. nasal process 

 B Maxillary process 



Med. nasal process 



Fig. 137. — Ventral view of head of human embryo of 8 weeks. His. 



segments — each upper, into arm, forearm and hand, each lower, into thigh, leg 

 and foot. 



The anlagen of the digits (fingers and toes) appear, during the fifth week, in 

 the broader, flattened distal portions of the limb buds. The boundaries be- 



