MESENCEPHALON 



DORSAL AORTA 



MESENCHYME 



VENTRAL AORTA 



Fig. 131. A section through the mesencephalon of a 33-hour chick embryo. 

 The foregut is a V-shaped cavity, and its ventral border is in contact with the 

 head epidermis. Above the foregut are the dorsal aortae and below, the ventral 

 aortae. The future aortic arches will connect these vessels. 



DORSAL AORTA 



DORSAL MESOCARDIUM 



EPIMYOCARDIUM 



ENDOCARDIUM 



Fig. 132. The heart region of a 33-hour chick embryo. The thick-walled 

 epimyocardium has closed in around the very thin-walled endocardium. Dorsally 

 the heart remains attached by the dorsal mesocardium. Ventrally it becomes freed 

 of the splanchnic mesoderm. 



SOMATIC LAYER OF 

 MESODERM 



SPLANCHNIC 



LAYER OF MESODERM 



DORSAL AORTA 



NEURAL TUBE 

 INTERMEDIATE MESODERM 



SOMATOPLEURE 

 SPLANCHNOPLEURE 



COELOM 

 NOTOCHORD 



Fig. 133. A section through a pair of somites. The somites are connected 

 with the lateral mesoderm by a strip of intermediate mesoderm. This latter forms 

 the pronephros, mesonephros, and metanephros. The lateral mesoderm of the 

 embryo proper is thick-walled and encloses the embryonic coelom. Farther out 

 the mesoderm becomes thin and forms part of the extraembryonic membranes. 



214 



