CAUSES OF TWINNING IN ARMADILLOS 119 



is the right. It would be interesting to know just how 

 general is the condition which Patterson describes. 



The preceding interpretation of twinning disposes, I 

 believe, of the lateral budding idea of Patterson. Certain 

 evidences favoring the fission theory have been presented 

 in passing, but the main evidences that twinning in the 

 armadillo is a case of longitudinal or bilateral fission 

 inhere in the facts brought out fully by the writer several 

 years ago in connection with closeness of resemblance 

 and with symmetry reversal. It was shown quite 

 conclusively that the resemblances between these twins 

 are as close as are the right and left sides of a single 

 individual, very closely approximating complete identity. 

 It was shown, in addition, that asymmetrical peculiarities 

 occurring on one side of one twin were very frequently 

 found on the opposite side of the other twin. If there 

 is any validity at all to this idea of symmetry reversal 

 it is obvious that it must have significance as the residue 

 of a former bilateral symmetry of the undivided embry- 

 onic axis, and it is equally obvious that the only 

 kind of division that would preserve such an original 

 symmetry and show it in mirror-imaging is the method 

 of bilateral fission. Lateral budding could never have 

 any such results. 



Stockard's theory of twinning in the armadillo is 

 based on Patterson's budding theory, and is therefore 

 open to the same objections. A few quotations from his 

 principal paper will reveal this point of view : 



The double primitive streaks in the hen's egg and other 

 forms all lend themselves to strengthen the interpretation that 

 double embryo-formation first asserts itself by a double gastrula- 

 tion or blastopore formation, which is initially a process of double 



