42 THE STUDY OF CHICK EMBRYOS 



Section Caudal to the Intestinal Portal (Fig. 35). The section is characterized: (i) 

 by the closing together of the neural folds to form the neural tube; (2) by the dorsal and 

 lateral folding of the entoderm, which, a few sections nearer the head end, forms the fore- 

 gut; (3) by the presence of the vitelline veins laterally between the entoderm and splanchnic 

 mesothelium; (4) by the wide separation of the somatic and splanchnic mesoderm and the 



. -Ectoderm 

 Descending aorta 

 Entoderm of fore-gut 

 Vitelline vein 



Entoderm 



FIG. 35. Transverse section caudal to the intestinal portal of a twenty-five-hour chick embryo. 



X 90. 



consequent increase in the size of the ccelom. In this region the ccelom later surrounds 

 the heart and forms the pleuro-pericardial cavity. 



The neural 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 ganglia. Mesodermal segments do not develop in this region; instead a diffuse 

 network of mesoderm partly fills the space between ectoderm, entoderm, and mesothe- 

 lium. This is termed mesenchyme and will be described later (p. 53). 



Neural tube 



Descending aorta 

 Notochord 



Splanchnic mesoderm 

 Epi-myocardiu m 



Ccdom 



Splanchnic mesoderm// 



Ectoderm 

 Somatic mesodert,. 

 Mesenchyma 



Fore-gut 



Endothelium of heart tube 



ntoderm 



FIG. 36. Transverse section through the intestinal portal of a twenty-five-hour chick embryo. 



X 90. 



Section through the Intestinal Portal (Fig. 36). This section passes through a vertical 

 fold of entoderm at the point where the latter is reflexed into the head as the 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. 



