78 ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 



left side of an otherwise normal developing embryo. This experi- 

 ment has been performed on the blastula of the newt, and resulted 

 in the production of situs inversus.^ Another method is to subject 

 the developing embryo to physical factors which are calculated to 

 affect the rate of activities of the tissues, and to direct these physical 



Fig. 33 



a, Dorsal ; b, Ventral ; c, Left side views of an embryo of Urodele to show the 

 position of the paired rudiments of the heart in the mesoderm beneath the 

 surface, d, Embryo from which the dorsal surface has been cut off and the entire 

 gut peeled out, thus revealing the ventral mesoderm, with the position of the 

 paired heart rudiments indicated by circles. The left rudiment, when isolated, 

 develops further than the right. (From Goerttler, Verb. Anat. Ges. xxxvii, 1928.) 



factors in such a way that one side of the embryo is affected more 

 than the other. This would appear to be why chick embryos show 

 situs inversus when they are locally damaged by overheating on the 

 left side during incubation. ^ Here, the intensity of action of the 

 tissues on the left side has been decreased, while in the previously 

 described experiment it is their amount which has been reduced. 

 ^ Wilhelmi, 1920, 1921. ^ Warynsky and Fol, 1884. 



