114 



THE PHYSIOLOGY OF TWINNING 



are merely the bulges due to the presence of the two 

 thickened areas, the medullary plates of the now twin 

 embryos. This is the true twin stage in armadillo 

 development. Before any further differentiation occurs, 

 however, these twin embryonic areas once more undergo 

 bilateral fission, rather less complete than the first 

 fission, for the median area does not, for a time at least, 



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Fig. 47. — An armadillo embryo in the true twin stage. The thick- 

 ened plates of ectoderm below the figures II and IV are the embryonic 

 primordia of the twin embryos and are as yet undivided to form the 

 quadruplet condition. Lettering same as in Fig. 46, (From Newman 

 after Patterson.) 



