EMBRYO OF SEVEN SEGMENTS 39 



gether, they constitute the angioblastira m which arises the extra-embr yonic 

 bl ood vascular syste m. The area pellucida has the form of the sole of a shoe with 

 broad toe directed forward. The head fold has become cylindrical and the head 

 of the embryo is free for a short distance from the germinal disc. The mesod erm 

 extend s on each sid e bevond _ thejiead Jej,ving _a median clear sgace ^ the proam- 

 niotic area ^ The e ntoderm is^rried forward in the head fold as the fore-gut, from 



Anterior neuropore 



Phai \n\ 



Le ft iiiteltine vlu , 



Mesodermal 

 segment 5 



Area pellucida 



Neural groove 



Notochoi d 



Area opaia 



Primitive streak 



Pi niutive knot 



Blood niand 



Fig. 33. — Dorsal view of a twenty-five-hour chick embryo with seven primitive segments. X 20. 



which later arise the pharynx, esophagus, stomach, and a portion of the small 

 intestine. The opening into the fore-gut faces caudad and is the fovea cardiaca. 

 The way in which the entoderm is folded up from the germinal disc and forward 

 into the head is shown well in a longitudinal section of an older embryo (Fig. 42). 

 The tubular heart lies ventral to the fore-gut and cranial to the fovea cardiaca. In 

 later stages it is bent to the right. Converging forward to the heart, on each side 

 of the fovea, are the vitelline veins, just making their appearance at this stage. 



