192 



O 



rotated ISO- 



EMBRYOLOGY 



SITUS INVERSUS 

 LARVA 



Fig. 115. Situs inversus in frog 

 tadpole. Left: Normal curvature of 

 heart and stomach of a tadpole. 

 Right: A section of the neural plate 

 plus the roof of the archenteron is 

 reversed in an early frog neurula. 

 Result: a reversal of the curvature of 

 the heart and stomach. 



Whereas the endoderm depends upon the mesoderm for some of its 

 structural characters, the splanchnic mesoderm in turn is induced by the 

 endoderm in the amphibian embryo. If the endoderm is removed in the early 

 neurula, the splanchnic mesoderm forms the same structures as the somatic 

 mesoderm. As a result, both splanchnic and somatic mesoderm develop 

 limbs. When the splanchnic layer is in contact with endoderm it forms 

 peritoneum and not limb. Since the splanchnic layer is normally in direct 

 contact with the endoderm and since the endoderm is necessary for its normal 

 differentiation, this relationship is another case of embryonic induction. 



These few facts make it very likely that the principles operating during 

 differentiation of the digestive system are very similar to those which have 

 been outlined for other parts of the embryo. Newer studies on the differentia- 

 tion of the endoderm are now in progress in several countries. 



