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 bhnd entodermal tubes, the fore- 

 gut and hind-gut respectively (Figs. 79 and 167 A). The region between these 

 intestinal tubes, open ventrally into the yolk sac, is sometimes termed the mid-gut. 



Pharyii. 



Hmd-(^ut 



Cloacal 

 membrane 

 AUanlois 



Cloaca 



Phayy}ix 



P/liiryi/^ca! 



n-cnibraiie 



Thyreoid 



gland 



Pericardial 



cavity 



Fore gill- 



Hepatic 

 diverticitlam 



Vol I; stalk 



Allantois 



Cloacal 



membrane 



Cloaca 



II ind-gut 



Fig. 167. — Diagrams showing in median sagittal section the human alimentary canal, pharyngeal 

 and cloacal membranes. X 35. A, 2 mm. embryo (modified after His); B, 2.5 mm. embryo (after 

 Thompson). 



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 



IS9 



