I20 THE PHYSIOLOGY OF TWINNING 



instead of single bud formation. Patterson's description of the 

 origin of the quadruplet buds in the Texas armadillo furnishes 

 the most striking case in the study of these conditions. And we 

 may conclude that the budding or accessory embryo-formation in 

 the egg of the armadillo is exactly the same developmental process 

 as that which gives rise to twins and double individuals in other 

 vertebrate eggs. 



In another place he says: "There is reason to believe 

 that, aside from the external factors discussed, the 

 armadillo egg is highly disposed toward the formation of 

 accessory embryonic buds.^' Again, in attempting to 

 explain why the deer, although it has a "period of 

 quiescence," fails to produce twins, he says: "The egg 

 of the deer may possess only a very slight tendency 

 toward accessory embryo formations.'" 



Exactly what does Stockard have in mind when he 

 uses the term accessory bud? It is clear that he uses 

 the term advisedly and means to imply just this: that 

 the original embryo retains its identity, but that, 

 through its loss of dominance over the rest of the blasto- 

 derm, accessory or secondary buds arise which give 

 rise to additional embryos. Evidently this is part and 

 parcel of the theory of budding, for a bud is essentially 

 an offshoot of a previously existing individual or of a 

 common stock. If, therefore, the phenomenon of twin- 

 ning in the armadillo turns out not to be budding at 

 all, but fission, the whole budding theory, together with 

 the causal theories based upon it, falls to the ground. 



