CHICK EMBRYO OF EIGHTEEN PRIMITIVE SEGMENTS 57 



reaching the optic vesicles they bend caudad, and as the paired descending aortae 

 may be traced to a point opposite the last primitive segments. In the region of 

 the fovea cardiaca they He close together and have fused to form a single vessel, 

 the dorsal aorta. They soon separate and opposite the last primitive segments 

 they are connected by numerous capillaries with the vascular network. In this 

 region at a later stage the trunks of the paired vitelline arteries will be differen- 

 tiated. The heart beats at this stage, the blood flows from the vascular area by 

 way of the vitelline veins to the heart, thence by the aortae and vitelline arteries 

 back again. This constitutes the vitelline circulation and through it the embryo 

 receives nutriment from the yolk for its future development. 



Jill 



Forehrain ■ J&MW «» r,, 



Bjgypffi ft ectoderm 



^^p^s^^ S^a Mesenc hymal 



ffi 





mm m 



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 Somafopleure ^^ 



1 



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 - — pleure 



Fig. 41. — Transverse section through the fore-brain of a thirty-six-hour chick embryo. X 75. 



In studying transverse sections of the embryo the student should not only 

 identify the structures seen, but should locate the level of each section by compar- 

 ing with Figs. 39 and 40, and trace the organs from section to section in the series. 



Transverse Section through the Fore-brain and Optic Vesicles. (Fig. 41). — 

 The optic stalks connect the optic vesicles laterally with the ventral portion of the fore- 

 brain. Dorsally the section passes through the mid-brain. We have alluded to the thickening 

 of the lens placode. Note that there is now a considerable amount of mesenchyma between the 

 ectoderm and the neural tube. In the germinal area the layers of mesoderm are present. 



Transverse Section through the Pharyngeal Membrane and Mid-brain (Fig. 

 42). — In the mid- ventral line the thickened ectoderm bends up into contact with the entoderm 

 of the rounded pharynx. At this point the oral opening will break through. On either side 

 of the pharynx a pair of large vessels are seen; the ventral pair are the ventral aorta. Two 

 sections cephalad their cavities open into those of the dorsal pair, the descending aorta:. The 

 section is thus just caudad to the point where the ventral aortae bend dorsad and caudad to 

 form the descending aortas. The section passes through the caudal end of the mesencephalon 

 which is here thick walled with an oval cavity.. Note the large amount of undifferentiated 



