80 DYNAMICS or HISTOGENESIS. II 



position so that the cephalic end was directed caudad and the caudal 

 end was directed cephalad. Tadpoles were reared from these experi- 

 mental embryos and in many cases a complete situs inversus viscerum 

 was presented. 



The transposition of the viscera is due to the fact that the reversed 

 segment presents a screw-like growth action of the epithelium oppo- 

 site to that normally present. The mesenchymal cells fonn a right- 

 handed spiral arrangement and in reacting to stretching cause a 90° 

 rotation of the gut from right to left, or counterclockwise. This 

 reversed rotation due to the torsional reaction of the mesenchyme 

 would cause the ventrally placed liver bud to be directed to the left 

 instead of to the right. 



An excellent discussion of the views of Wilder, Bateson, and New- 

 man as regards mirror imaging and symmetry reversal in monsters 

 will be found in a paper by Morrill. That Morrill had a clear-cut 

 perception of the location in which solution of the problem of nor- 

 mal asymmetry of the xdscera was to be found is seen in the follow- 

 ing statement: "the factors controlling asymmetry are located in 

 the primitive gut and become operative before the liver bud has 

 developed." 



Crampton suggested that asymmetry in the viscera may depend 

 upon spiral cleavage during the first stages of development. This 

 rested on the fact that the majority of snails possess a dextral shell 

 associated with a right-handed spiral cleavage. Certain snails have 

 a sinistral shell and reversed asymmetry in the \'iscera. This rever- 

 sal of the viscera was associated with a left-handed spiral cleavage. 



From a study of vertebrate monsters, however, Morrill correctly 

 comes to the following conclusion: "From the evidence at hand, it 

 seems probable that the primary cause of \dsceral as\TTimetn,^ in verte- 

 brates is to be sought for at the completion of cleavage rather than in 

 the period of cleavage itself." 



Origin of Spiral Growth. 



The explanation of spiral epithelial growth is under investigation. 

 Spiral growth involves two motions: a motion of rotation, circular, 

 and a motion of straight progression, rectilinear. The latter is evi- 

 dently imparted from growing points of mitosis. Growth would 



