TWINNING IN OTHER SPECIES OF ARMADILLO 79 



in the small, simple uterus that their external membranes 

 fused together so as to simulate a monochorial condition. 

 It is certainly a strange circumstance that within a 

 small group of closely related and sharply differentiated 

 forms like the armadillos there should occur two methods 

 of twinning so diametrically opposite in character: 

 the splitting up, as in Dasypus, of a single egg into 

 several embryos, and the intimate fusion, as in Euphrac- 

 tus, of originally separate eggs into one vesicle. Stranger 

 still is the fact that the end-results of the two processes 

 are so strikingly similar as to lead one to beheve that 

 they are the result of the same fundamental processes. 

 Such a finding as this should indicate the necessity of 

 caution in interpreting the various types of monochorial 

 conditions in human twins, for it seems highly probable 

 that the common chorion in many observed cases may, 

 as in Euphractus, be due merely to a secondary fusion of 

 originally separate membranes. 



Another interesting discovery is that in Euphractus 

 villosus occasional cases of uniparous births occur. 

 Fernandez found five such cases in thirty-:^ur pregnant 

 uteri. Three interpretations of this condition are 

 obviously possible. There may be: (a) a certain 

 amount of prenatal mortahty of one twin of a pair; 

 (6) a failure to ovulate on the part of one of the ovaries; 

 or (c) one of the eggs may fail to be fertihzed. Other 

 reasons might be suggested, but since Fernandez does 

 not furnish the crucial data necessary for deciding the 

 matter — a statement as to whether in these cases of 

 single embryos one or two corpora lutea occur — it 

 would be useless to speculate further. It seems strange 

 that one who knows so well the unique value of the 



