DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 143 



head. The extreme end of the head region is bent ventrally almost at a right 

 angle to the long axis of the body, the bend being known as the cephalic flexure. 

 On the ventral side of the body and cranial to the attachment of the yolk sac 

 there is a rather large protrusion which indicates the position of the heart. 

 Between the protrusion and the bent part of the head there is a deep depres- 

 sion the oral fossa. A series of bilaterally symmetrical structures appear in 

 the body region along the sides of the neural tube. These are the primitive 

 segments (mesodermic somites). 



All these features are even more clearly shown in Fig. 120, which represents 



Cephalic 

 flexure 



Maxillary process 



Oral fossa 



Branchial groove I 



Branchial arch II ^ A " Mandibular process 



Ventral aortic trunk 



Primitive 

 segments 



Umbilical vein 



Naso-frontal process 



Belly stalk 

 Sacral flexure 



FIG. 122. Human embryo of the third week. His. 



an embryo 2.5 mm. in length. There is also a further increase in the size of 

 the head region. A distinct concavity, caused by the dorsal flexure, is seen in 

 the dorsum of the embryo. 



Another embryo, apparently older but only 2.15 mm. long, shows a re- 

 markable exaggeration of the dorsal flexure (Fig. 121). The middle part of the 

 body seems to be drawn ventrally by the yolk sac. While this may be a 

 normal feature at this stage, it soon disappears and the concavity becomes a 

 convexity (see p. 144). A new feature also appears in this embryo in the form 

 of two vertical depressions just caudal to the head region. These depressions 



