EMBRYOLOGY 



Fourth Week. The embryo is markedly curved on itself, and when viewed in profile is almost 

 circular in outline. The cerebral hemispheres appear as hollow buds, and the elevations whici 

 form the rudiments of the auricula are visible. The limbs now appear as oval flattened projec- 

 tions (Fig. 60). 



Heart 



Hyoid arch 

 Mandibular arch 

 Maxillary process 



Eye 



Fore-limb 



Hind-limb 

 FIG. 61. Human embryo, thirty-one to thirty-four days old. (His.) 



Fifth Week. The embryo is less curved and the head is relatively of large size. Differentiation 

 of the limbs into their segments occurs. The nose forms a short, flattened projection. The cloacal 

 tubercle is evident (Fig. 61). 



Eye s 



Auricula 



Fore-limb 



Hind-limb 



Umbilical cord 



FIG. 62. Human embryo of about six weeks. 

 (His.) 



FIG. 63. Human embryo about eight and a half 

 weeks old. (His.) 



Sixth Week. The curvature of the embryo is further diminished. The branchial grooves 

 except the first have disappeared, and the rudiments of the fingers and toes can be recognized 

 (Fig. 62). 



Seventh and Eighth Weeks. The flexure of the head is gradually reduced and the neck is 

 somewhat lengthened. The upper lip is completed and the nose is more prominent. The nostrils 



