Ill DIFFERENTIATION 111 



otherwise have had a very different fate. The normal vesicle 

 and capsule are developed alongside the transplant. It is 

 even possible to graft a vesicle of Rana on to a larva of 

 Amblystoma. The foreign vesicle then becomes enveloped 

 in a periotic derived from Amblystoma cells. The normal 

 Amblystoma labyrinth is present as well (Spemann, Lewis, 

 Streeter). 



Our last example is furnished by the experimental situs 

 inversus viscerum et cordis produced by Spemann and 

 Pressler in Amphibian larvae (Rana and Bombinator). A 

 square piece is cut out of the back of the embryo in the medullary 

 fold stage, turned right round so that its anterior end faces 

 posteriorly and its right side to the left, and grafted in again. 

 The part removed involves nervous system, notochord, some 

 mesoderm, and the roof of the gut, but not the floor of the 

 latter nor of course the heart or any subjacent structures. 

 The embryo continues to develop, and shows inverted viscera 

 and inverted heart. The part of the gut removed and inverted 

 is in the region of the duodenum, but before the dorsal 

 pancreas was developed ; nevertheless, not only is the dorsal 

 part of the intestine inverted, but the whole of the alimentary 

 canal, that is, parts posterior and ventral to that which was 

 turned round. In other words, the liver is pushed over to the 

 left instead of to the right, the dorsal and ventral pancreas 

 unite on the left instead of on the right, the stomach is on 

 the right and the duodenum on the left, while the intestine 

 passes into the rectum on the right. Moreover, the heart is 

 inverted. The right is larger than the left vitelline vein, and 

 the ventricle is bent to the left, the left auricle is larger than 

 the right. The spiral valve starts at the right instead of at 

 the left side of the truncus, so that the cavum aorticum is on 

 the left, the cavum pulmonale on the right. The heart and 

 viscera of the tadpoles operated on are the mirror images 

 of the normal organs, the situs inversus viscerum et cordis is 

 complete, and it is clear that an experimental alteration of 

 the normal position of some parts has incited a corresponding 

 change in other parts which were not immediately touched 

 in the operation. 



