DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



145 



berance forms the anlage of the upper extremity. This is known as the upper 

 limb bud. A similar protuberance caudal to the sacral flexure is the lower limb 

 bud. 



Fig. 124 shows a stage slightly further advanced than Fig. 1 23 . The embryo 

 as a whole is more stocky, and the head is still larger in proportion to the rest 

 of the body. This feature is especially noticeable from this stage up to the 

 time of birth. The sacral and cervical flexures are still very prominent. The 



Cervical Cervical 



depression flexure 



Dorsal flex 



Branchial arch IV 

 Branchial groove III 

 Branchial arch III 

 Branchial groove II 

 Branchial arch II 

 Branchial groove I 



Branchial arch I 



Mandibular process 

 Maxillary process 

 Eye 



Nasal pit 



T } Heart 



Yolk stalk 



Lower limb bud 



Primitive segments 



Upper limb bud Liver Sacral flexure 

 FIG. 123. Human embryo with twenty-seven pairs of primitive segments (7 mm., 26 days). Mall. 



dorsal flexure, however, is less prominent and the body of the embryo is more 

 nearly straight. The sacral and cervical flexures from this time on become 

 more and more reduced, while the cephalic flexure, which primarily affects the 

 embryonic brain, persists as the mid-brain flexure in the adult. 



The branchial arches are actually no smaller but appear less prominent. 

 Between the mandibular process and the maxillary process there is a distinct 

 notch which corresponds to the angle of the mouth. The second arch has 

 enlarged at the expense of the third and fourth, has grown back over them to a 



