THE HINDGUT 191 



MESONEPHRIC DUCT 

 CLOACA I _ RECTUM 



X— POSTERIOR INTESTINAL PORTAL 

 INTESTINAL CAECUM 



Fig. 114. The hindgut and its derivatives. The hindgut communicates with 

 the yolk through the posterior intestinal portal. The cloaca receives the meso- 

 nephric duct. From the floor of the hindgut a rapidly growing sac develops into 

 the endodermal component of the allantois. A small intestinal caecum forms 

 from a ventral outgrowth of the hindgut. 



allantois grows around the inner surface of the shell and thus its blood 

 capillaries come in direct contact with the air which diffuses through the 

 porous shell and shell membranes. 



Although we know much less about the principles governing the develop- 

 ment of the digestive system, as compared with the nervous system, a few 

 facts are clear. First, the early differentiation of the endoderm is independent 

 of the nervous system. In the amphibian egg the endoderm shows a high 

 degree of differentiation in exogastrulae, in which no nervous system forms. 

 Second, the normal asymmetry of the stomach and intestines is dependent 

 upon the roof of the archenteron in the frog neurula. If a combined section 

 of the roof of the archenteron and neural plate is cut out and rotated 180°, 

 the resultant tadpole shows reversed asymmetry, or situs inversus — that is, 

 the position of the stomach is reversed, and the heart is on the opposite side 

 of the body (Fig. 115). Third, the formation of an endodermal tube is 

 dependent upon mesoderm cells. When either lung or intestinal epithelium 

 is grown in tissue culture, the cells spread out in a flat plate. There is no 

 morphological difference between lung and intestinal cells. If a culture of 

 connective tissue cells is now added to these cultures, the epithelial cells 

 form tubes which take on the characteristics of lung or intestine respectively. 

 Other epithelial tissues behave in a similar fashion. It is clear that some 

 mesoderm is necessary for the formation of characteristic tubules. 



