DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 115 



Figure 89 shows an embryo of n mm. All the flexures are slightly 

 reduced except the cephalic. The cephalic flexure, which primarily affects 

 the embryonic brain, persists as the mid-brain flexure of the adult. Two 

 slight concavities have appeared in the dorsal profile, the occipital depres- 

 sion and the cervical depression. The latter becomes more conspicuous 

 as development proceeds and persists as the depression at the back of the 

 neck in the adult. The first branchial arch is a strong feature of the head, 

 the maxillary process being especially prominent. This process has grown 

 forward to form intimate contact with the nasal region. The second arch 

 now hides the third and fourth arches, and the depression behind the second 



Cervical flexure 

 Occipital depression 



Cervical depression 



Dorsal flexure 



Umbilical cord 



X/f 



Sacral flexure 

 FIG. 89. Human embryo n mm. long (31-34 days). His. 



is known as the precermcal sinus. The first groove can be more readily 

 appreciated as the site of the external auditory meatus, as can also the 

 surrounding parts of the first and second arches be better appreciated as 

 rudiments of the concha. The distal part of the fore-limb bud is flattened 

 like a paddle, and the radial depressions in it mark the boundaries between 

 the digits. In the proximal portion the fore-arm and arm are faintly in- 

 dicated. The hind-limb bud is divided by a constriction into a proximal and 

 distal portion; the latter is the beginning of the foot. During development 

 the fore-limb is always at a slightly more advanced stage than the hind-limb. 

 The ventral rotundity of the body is pronounced. 



In an embryo measuring 15.5 mm. (Fig. 90) the dorsal flexure is much 

 reduced and the axis of the body is approaching the definitive line. The 



