220 



EMBRYOLOGY 



continuous cavity from the diencephalon through the optic stalk and into the 

 optic cup. Most of the other structures are in about the same relationship 

 as in Figure 137. 



In a still more posterior section the very beginnings of the olfactory 

 placodes may be seen opposite the telencephalon (Fig. 139). Like the lens 

 of the eye and the otocyst, they form as thickenings of the epidermis and 

 invaginate. They do not, however, separate from the adjacent epidermis. 



The circulatory system in this region consists of the bulbus arteriosus, 

 with its thick-walled epimyocardmm and its thin-walled endothelium. A 

 short ventral aorta communicates (in other sections) with the third aortic 

 arches and these in turn run up through the visceral arches to join the dorsal 

 aortae. The cardinal veins are more ventral and lateral compared to their 

 position in previously described sections. From the floor of the pharynx a 

 clear, V-shaped thyroid evagination is seen. Mesodermal somites are present 

 opposite the myelencephalon. 



The section shown in Figure 140 passes through the developing liver 

 buds. These are outgrowths of the wall of the gut at the region where the 

 vitelline veins join before entering the sinus venosus of the heart. The liver 



EPIMYOCARDIUM 

 OLFACTORY PLACODE 



TELENCEPHALON 



ENDOCARDIUM 



ANTERIOR CARDINAL VEIN 



DORSAL AORTA 



THYROID 



Fig. 139. The olfactory placodes, bulbus arteriosus, and thyroid evaginat 

 of a 48-hour chick embryo. 



ion 



FOREGUT 

 EMBRYONIC COELOM 



VENTRICLE — — 



VITELLINE VEIN 



Fig. 140. The vitelline vein and the liver of a 48-hour chick embryo. 



