CAUSES OF TWINNING IN ARMADILLOS 103 



mordia were physiologically isolated on the originally 

 single ectodermic vesicle. From the time of their iso- 

 lation till they are born the four individuals remain 

 morphologically and physiologically independent. They 

 are merely inclosed in a common chorion, which ante- 

 dates the twinning process. 



For a considerable time then I have steadfastly held 

 the view that twinning in the armadillo is caused by 

 arrested development resulting in a more or less complete 

 obliteration of the axiate organization, together with 

 loss of the integrative properties of the original head 

 and the emancipation of subordinate regions from the 

 control of the original dominant region. This plainly 

 suggests physiological isolation. I have no reason to 

 abandon this general view, but shall attempt to give 

 to it a more concrete setting. 



CAUSES OF THE '' PERIOD OF QUIESCENCE" 



The progress of a scientific theory is one that pro- 

 ceeds step by step from immediate cause to causes more 

 and more remote. The immediate cause of twinning in 

 the armadillo is the physiological isolation of secondary 

 growing-points (apical points) ; the cause of physiological 

 isolation is the partial obliteration of the axiate relations 

 in the ectodermic vesicle; the cause of this deaxiation 

 is the greatly lowered rate of development incident to 

 the ''period of quiescence." The next link in our chain 

 of causes is the one that will account for the "period of 

 quiescence." In his voluminous monograph on Develop- 

 mental Rate and Structural Expression, Stockard (192 1) 

 reviews the armadillo situation and offers "an explana- 

 tion of polyembryony in the armadillo," which involves 



