THE STUDY OF CHICK EMBRYOS 



as blood islands. Together, they constitute the angioblast from which 

 arises the extra-embryonic blood vascular system. The area pellucida 

 has the form of a slipper, 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 mesoderm extends on each side 

 beyond the head leaving a median clear space, the proamniotic area. 



Anterior neuropore 



Fore-brain 



Free portion of head 



Pharynx 



Left vitelline vein* 



Mesodcnnal 



Area pellucida 



Intestinal porta 



igM mtelline 

 vein 



.yrtf 4r Neural groove 



.-. a 



Segmenlal 

 zone 



~Primitive knot 



f- Blood island 



Notochord 



Area opaca 



Primitive streak 



FIG. 33. Dorsal view of a twenty-five-hour chick embryo with seven primitive segments. 



X 20. 



The entoderm is carried forward in the head fold as the fore-gut, from 

 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 

 intestinal portal. The way in which the entoderm is folded up from the 

 blastoderm 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 intestinal portal. In later stages it is 

 bent to the right. Converging forward to the heart, on each side of the 



