110 



TEXT-BOOK OP EMBRYOLOGY. 



body wall is closed comprise the differentiation of the head, neck and trunk 

 regions and the development of the extremities as appendages of the trunk. 

 Even in Eternod's embryo (Fig. 83) the region where the brain is developing 

 is greater in diameter than the other part of the embryo. Thus the begin- 

 ning of the head is indicated by an increase in size due primarily to the growth 

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

 angle to the long axis of the embryo, the bend occurring in the mid-brain 

 and being known as the cephalic flexure. This is the first of the flexures 



Heart 



Ant. entrance to 

 prim, gut (Ant. 

 intest. portal) 



Post, entrance to 

 prim, gut (Post, 

 intest. portal) 



Cerebral plate 



Amnion 



Yolk sac 

 (cut edgej 



Yolk sac 



Neural tube 



Primiti\ e 

 segment 



Neural fold 

 Neural groove 



Belly stalk - 



a b 



FIG. 83. (a) Ventral view; (b) dorsal view of human embryo with 8 pairs of mesodermal somites 



(2.11 mm.). Eternod. From models by Ziegler. 



In b the amnion has been removed, merely the cut edge showing; in a the yolk sac has 



been removed. 



that appear as development proceeds. On the cephalic side of the yolk sac 

 attachment is a protrusion which indicates the position of the heart in what 

 now may be called the cervical region or neck. Between the protrusion 

 caused by the heart and the fore-brain there is~a depression which fore- 

 shadows the oral and nasal cavities and is now called the oral fossa . 



In Fig. 84, showing the dorso-lateral aspect of an embryo 2.5 mm. long 

 and possessing 14 primitive segments, the beginning of the head, the cephalic 



