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 cranial and caudal to the spherical vitelline sac, blind 

 entodermal tubes, the fore- gut and kind-gut respectively (Figs. 79, 255 and 

 167,4). The region between these intestinal tubes, open ventrally into 



Pharynx 



Pharyngeal 

 membrane 



Cloacal 

 membrane 

 Allantois 



Cloaca 



FIG. 167. Diagrams showing in median sagittal section the human alimentary canal. X 35- 

 A, 2 mm. embryo (modified after His); B, 2.5 mm. embryo (after Thompson). 



the yolk sac, is sometimes termed the mid-gut. As the embryo and the 

 yolk sac at first grow more rapidly than the connecting region between 

 them, this region is apparently constricted and becomes the yolk stalk, 

 or vitelline duct. At either end the entoderm comes into contact ven- 

 trally with the ectoderm. Thus there are formed the pharyngeal membrane 

 of the foregut and the cloacal membrance of the hind-gut. In 2 mm. 

 11 161 



