176 THE PHYSIOLOGY OF TWINNING 



of a blastoderm. The amphibian embryo has a high capa- 

 city for regeneration and hence a very large proportion of 

 the half-embryos regenerated so as to produce each a 

 whole individual. As a rule, however, the regenerated 

 half did not grow so rapidly nor differentiate so completely 

 as the older half. The result is that the right side of 

 the embryo derived from the left-hand piece and the 

 left side of the embryo derived from the right-hand 

 piece were usually imperfect in various ways. We may 

 speak of the regenerated side of both twins as the inner 

 side. Now it happens that on the inner side the append- 

 ages are always smaller, the eyes sometimes smaller, 

 and the body musculature much less developed. As a 

 consequence of the lack of body musculature the bodies 

 of the twin embryos are often concave on the inner side 

 and may be even spirally coiled. In this way it often 

 happens that structures on the two outer sides are similar 

 and those on the two inner sides equally similar. This 

 is a sort of mirror-image asymmetry that might be 

 readily explained as the result of imperfect regeneration 

 of the side that had been lost; but it is not this rather 

 obvious sort of mirror-imaging that attracted the interest 

 of these authors. The really striking discovery was that 

 out of the thirty right-hand pieces that survived exactly 

 half grew into larvae that showed situs inversus viscerum; 

 the other half showed normal left-hand asymmetry. 

 This discovery is rendered even more striking by the 

 fact that out of twenty-five surviving left-hand pieces 

 none showed definite reversed or right-hand symmetry. 

 One left-hand piece there was in which a slight irregu- 

 larity of the heart was noted, that might have been 

 interpreted as a case of minimal reversed symmetry. 



