EMBRYO OF SEVEN SEGMENTS 



41 



The neviral tube at this level forms the third brain vesicle or hind-brain. The neural 

 folds have not yet fused and at their dorsal angles are the neural crests, the anlages of the 

 spinal gangUa. Mesodermal segments do not develop in this region; instead a diffuse net- 

 work of mesoderm partly fills the space between ectoderm, entoderm, and mesothehum. This 

 is termed mesenchyme and will be described later. 



Transverse Section through the Fovea Cardiaca (Fig. 36).— This section passes 

 through a vertical fold of entoderm at the point where the latter is reflexed into the head as the 



Neural crest 

 Neural tube- 



Ectoderm 



Notochord \ '%'.} 



%% ^^ 



Myocarditim. 



-Entoderm of fore-gut 

 -Vitellim vein 



Entoderm 



Fig. 35. — Transverse section caudal to the fovea cardiaca of a twenty-five-hour chick embryo. X 90. 



fore-gut (cf . Fig. 42) . The entoderm forms a continuous mass of tissue between the vitelline 

 veins thereby closing the fore-gut ventrally. The splanchnic mesoderm is differentiated into 

 a thick- walled pouch on each side, lateral to the endothelial layer of the veins. 



Transverse Section through the Heart (Fig. 37). — Passing cephalad in the series 

 of sections the vitelline veins open into the heart just in front of the fovea cardiaca. The 

 entoderm in the head fold now forms the crescentic pharynx or fore-gut, separated by the heart 

 and splanchnic mesothehum from the entoderm of the germinal disc. The descending 



Neural tube 



Splanchnic mesoderm 

 {myocardium) 



Ectoderm 

 Somatic mesoderm 



Mesenckymu 

 Fore-gut 



Endothelium of heart tube 

 Entoderm 



Fio. 36.- 



Splanchnic mesoderm ij 

 -Transverse section through the fovea cardiaca of a twenty-five-hour chick embryo. 



X 90. 



aortse are larger, forming conspicuous spaces between the neural tube (hind-brain) and the 

 pharynx. The heart, as will be seen, is formed by the union of two endothelial tubes, similar 

 to those constituting the vitelhne veins in the preceding sections. The median walls of these 

 tubes disappear at a shghtly later stage to form a single tube, the endocardium. Thickened 

 layers of splanchnic mesoderm, which, in the preceding section, invested the vitelhne veins 

 laterally, now form the mesothelial wall of the heart. In the median ventral hne, the layers 

 of splanchnic mesoderm of each side have fused and separated from the splanchnic mesothe- 



