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EMBRYOLOGICAL TYPES 



being supplied by blood-vessels which run out from the embryo 

 along the allantoic stalk. As the shell is porous, the blood- 

 vessels of the allantois form a region where oxygen is taken 



Fig. ioo. — Gallus : view of an embryo chick after four days' incubation, 

 from the right side. 

 aa, arterial arches ; ac, anterior cardinal ; al, allantois ; au, auricular 

 portion of the heart ; cf, choroid fissure of the optic cup ; da, dorsal aorta ; 

 dC, ductus Cuvieri ; dv, ductus venosus ; e, optic cup ; ic, internal carotid 

 artery ; /, lens ; mb, midbrain ; pc, posterior cardinal ; t, tail ; ua, umbilical 

 artery ; uv, umbilical vein ; v, ventricular portion of the heart ; va, vitelline 

 artery ; vv, vitelline vein. 



into and carbon dioxide is given off from the blood. The 

 allantois therefore functions as a respiratory organ, and is of 

 the highest importance. The gill-slits do not function as 



