EMBRYO OF SEVEN SEGMENTS 



51 



this region, instead a diffuse network of mesoderm partly fills the space between 

 ectoderm, entoderm and mesothelium. This is termed mesenchyma and will 

 be described later. 



Neural plate 

 Ectoderm 

 Somatic mesoderm 



Splanchnic mesoderm 



descending aorta 



Neural groove 



Mesodermal Segment 

 Coelom 



Notochord 

 Entoderm 



Fig. 33. — Transverse section through the fifth pair of mesodermal segments of a twenty-five-hour chick 



embryo. X 90. 



Mured crest- 



Neuraltube 



Notochord 



Alyoeardium- 

 Entoderm 



Ectoderm 

 Descending aorta. 



Entoderm of Fortqut 



Vitelline vein 



Fig. 34. — Transverse section caudal to the fovea cardiaca of a twenty-five-hour chick embryo. X 90. 



Neural tube 



Descending aorta — % 



Notochord 



Splanchnic mesoderm 

 (Myocardium) 



Ccelom - 



Ectoderm 

 Som. mesoderm 



Mesenchyma. 



Endothelium of 

 heart tube 



Entoderm 



Splanchnic mesoderm // 

 FlG. 35. — Transverse section through the fovea cardiaca of a twenty-five-hour chick embryo. X 90. 



Transverse Section through the Fovea Cardiaca (Fig. 35). — This section is 

 marked by a vertical layer of the entoderm at the point where it is folded into 

 the head as the fore-gut. The entoderm is thickened laterally and forms a 

 continuous mass of tissue between the vitelline veins. The splanchnic mesoderm 



