EBEN J. CAREY 79 



This loop may be produced with a string or elastic band. With 

 one end between the left thumb and index finger directed above and 

 the lower element between the right thumb and index finger directed 

 below we may exemplify the small and large intestines, respectively. 

 By rotating the thumb and index finger clockwise, the string or band 

 is wound as a left-handed helix. After this slight initial torsion, 

 elongation of the intestine may be exemplified by approximating 

 the left and right hand. A looping of the segment of the string in 

 the right hand over that of the left is an inevitable reaction due to 

 the direction of the torsion. A reverse looping may be produced by 

 winding in the opposite direction or by excessive torsion in the previous 

 direction. 



The direction of epithelial growth is counterclockwise or as a left- 

 handed helix. The elastic mesenchyme is drawn out in a correspond- 

 ing direction. The resultant reaction of the mesenchyme is in the 

 reverse direction from left to right. This is an inevitable reaction of 

 the mesenchyme to the active torsion produced by the dominant epi- 

 thelial growth. The torsion of the digestive tube, therefore, is a 

 mesenchymal reaction to the torsional action of growth of the epi- 

 thelial tube. 



Situs Inversus Viscerum. 



The normal asymmetry of the abdominal viscera as well as the 

 position of the gut is dependent upon the action of the spiral growth 

 of the epithelial tube and the resultant reaction of the mesenchyme. 

 The left-handed helicoidal growth of the epithelium causes a corre- 

 'sponding arrangement of the cells of the less actively growing mesen- 

 chyme. The cells of the latter are elongated or stretched. Subse- 

 quently these mesenchymal cells react in the opposite direction from 

 left to right. This causes the 90° rotation of the gut in a clockwise 

 direction. The ventral position of the liver bud prior to rotation is 

 now directed to the right comparable to the 90° displacement dorsally 

 of the right vagus nerve after rotation is completed. 



Pressler produced experimentaly situs inversus in Bombinator. 

 While the embryo was in the neurula stage Pressler exsected a quadri- 

 lateral piece of the medullary plate plus a part of the dorsal surface 

 of the primitive gut. This piece was then replaced in the reversed 



