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TEXT-BOOK OF EMBRYOLOGY. 



the light of more recent studies probably being too low). The body is more 

 robust than in the preceding stage. In addition to the cephalic flexure the 

 dorsum in profile is a curve, with three rather prominent regions of curvature; 

 a cervical flexure, a dorsal flexure and a sacral flexure. The whole embryo is 

 slightly twisted around its long axis, the head turned toward the left and the 

 caudal end toward the right. In the cervical region are three vertical de- 

 pressions which diminish in size from before backward. Alternating with 

 these are prominences which also diminish from before backward. These 

 alternating depressions and prominences are the branchial grooves and arches 



Mid-brain flexure 



Eye 



Maxillary process 



Heart 



Hind limb bud 



Fore limb bud Umbilical cord 



FIG. 86. Human embryo of 4 mm. Rabl, from Kollfnan's Atlas. 



which are homologues of the gill slits and gill bars in fishes. The first arch 

 lies in front of the first groove and bounds the oral fossa laterally; its two 

 subdivisions, the mandibular process and maxillary process, with the notch 

 between representing the future angle of the mouth, are already differentiated. 

 Through the development of the first arch the depth of the oral fossa is 

 considerably increased. The heart causes a conspicuous protrusion on the 

 ventral side of the cervical region. The constriction between the body of 

 the embryo and the yolk sac is marked, and this attenuated portion of the 

 yolk sac is from now on spoken of as the yolk stalk. The structure attached 

 caudal the yolk stalk and turned over the right side of the embryo is the belly 

 stalk which later will be included in the umbilical cord. 



