CHAPTER VII 



THE ENTODERMAL CANAL AND ITS DERIVATIVES : THE BODY CAVITIES 



When the head- and tail-folds of the embryo develop, there are formed both 

 cranially and caudally from the spherical vitelline sac blind entodermal tubes, 

 the fore-gut and hind-gut respectively (Fig. 161 A). The region between these 



Hind-gut 



Cloaca! 



membrane 



AUanlois 



Cloaca 



Pharynx 



Pharyngeal 



membrane 



Thyreoid 



gland 



Pericardial 



cavity 



pore-gut 



Hepatic 

 diverticulum 



Yolk-stalk 



AUanlois 



Clnaral / 



membrane 

 Cloaca 



Hind -gut 



Fig. 161. — Diagrams showing in median sagittal section the alimentary canal, pharyngeal and cloacal 

 membranes. .1, 2 mm. embryo, modified after His; B, 2.5 mm. embryo, after Thompson. 



intestinal tubes, open ventrally into the yolk-sac, is known as the mid-gut. As 

 the embryo and the yolk-sac at first grow more rapidly than the connecting re- 

 gion between them, this region is apparently constricted and becomes the yolk- 

 stalk, or vitelline duel. At either end the entoderm comes into contact ventrally 

 with the ectoderm. Thus there are formed the pharyngeal membrane of the fore- 



168 



