DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 147 



just caudal to the cervical flexure. The cervical depression becomes more con- 

 spicuous in later stages and finally persists as the depression at the back of the 

 neck in the adult. 



The maxillary process is more prominent than in the preceding stages, as is 

 also the naso-optic furrow. The second arch has become larger and has grown 

 over the third and fourth, thus completely hiding them, but a depression known 

 as the precervical sinus is left just caudal to the second arch. The first branch- 

 ial groove is relatively large and marks the site of the external auditory meatus, 

 while the surrounding portions of the first and second arches in part are 

 destined to give rise to the external ear. 



Cervical flexure 

 Occipital depression 



. 



Cephalic flexure 



Dorsal flexure 



Umbilical cord 



Sacral flexure 

 FIG. 125. Human embryo n mm. long (31-34 days). His 



The distal portion of the upper limb bud has become flattened, and four 

 radial depressions mark the boundaries between the digits. The lower limb 

 bud is now divided by means of a constriction into a proximal and a distal 

 portion. In development the upper limb is always slightly in advance of the 

 lower. 



The rotundity of the abdomen, due to the rapidly growing heart and liver, 

 is more pronounced than in the preceding stages. 



Fig. 126 shows a stage in which the crescentic form of the body, as seen in 

 profile, is not so apparent. This is* due principally to the partial straightening 

 of the cervical flexure and to the greater rotundity of the abdomen. The 



