auditor/ vesicle 

 bindbrain 

 precardlnal 



55- hour chick 



omnionic cavity 



midbrain 

 -forebrain 



lens vesicle 



optic cup 



first aortic loop 



ventral aorta 

 qill slit 

 ventricle 

 auricle 



dorsal aorta 

 nerve cord 



sero-amnionic cavity 



ectoderrr? 



amnion 

 s,nerve cord 



x-section ot A. 



endoderm 



-amnionic fold 

 -Vitelline vein 



vitelline artery 

 somite 



toil bud 



x-section at B. 



Fig. 23-8. The beginnings of the various organ systems are well marked off in the 55-hour chick. The figure to the 

 left is of the dorsal view. Note that the head is turned to the right as it lies on top of the yolk mass. Two cross- 

 sections are also shown: the upper figure is taken through the region marked A; and the lower, through the region 

 marked B. Note how the extraembryonic membranes are formed. 



