ORGANIC SYMMETRY IN ARMADILLO QUADRUPLETS. 2O3 



of a system of symmetry common to both individuals, whether 

 they are joined or separate. 



In closing it may be of interest to state that an examination of a 

 considerable number of sets of quadruplets reveals no indication 

 of a reversal of symmetry in the viscera. As in human duplicate 

 twins symmetry reversals are confined to the integumentary 

 structures. Why should this be so? Bateson propounds this 

 inquiry for duplicate twins as follows: "If anyone could show 

 how it is that neither of a pair has transposition of the viscera 

 the whole mystery of division would, I expect, be greatly illu- 

 minated." Now in the armadillo the process of polyembryonic 

 budding that results in twinning is initiated and carried out in 

 the ectoderm, while the endoderm becomes involved only passively 

 and considerably later. Here we probably have the answer to 

 Bateson's inquiry, for symmetry reversals involve only the tissues 

 that are primarily concerned in twinning. How much this state- 

 ment serves to illuminate the mystery of division I am not pre- 

 pared to say. 



I am inclined to believe that duplicate human twins become 

 physiologically isolated at a considerably earlier period than do 

 armadillo quadruplets, and my reason for this belief is founded 

 on the fact that there is so very little mirror-imaging in the former 

 and so much in the latter. It appears to be a good general rule 

 that the earlier the separation the more complete is the reorgan- 

 ization of symmetry relations in the separated individuals and 

 the less residuum of the original common symmetry. Double 

 monsters doubtless begin to separate comparatively late in on- 

 togeny and hence show very pronounced mirror-imaging. Arm- 

 adillo quadruplets appear to exhibit a condition intermediate 

 between duplicate twins and double monsters for they have an 

 interesting combination of the effects of an original common 

 system of symmetry and of secondary and tertiary systems of 

 symmetry. Since it seems entirely probable that human dupli- 

 cate twins are separated at a much earlier period than are arma- 

 dillo quadruplets, it may not be unreasonable to look for this 

 separation at some period of cleavage. Or there may be a 

 division of the inner cell mass into the primordia of two embryos. 

 The problem of the exact mode of origin of duplicate human twins 

 is however likely to remain unsolved for a long time to come. 



