CAUSES OF TWINNING IN ARMADILLOS lOi 



of this phenomenon. In a general paper on the natural 

 history of the nine-banded armadillo the view was 

 expressed that the process of twinning was due to a 

 lowering of the rate of metabolism of the early embryonic 

 vesicle, resulting in the ''physiological isolation of parts 

 at certain distances from the dominant (apical) region. 

 When such isolation occurs new centers of control arise, 

 which produce buds capable of establishing whole new 

 systems like the original." At that time no facts were 

 available which seemed to account for the lowering 

 of the rate of development of the embryonic vesicle. 

 Certain peculiar bodies, that were identified by a well- 

 known protozoologist as protozoan parasites, were found 

 abundantly in ovarian oocytes, and the suggestion 

 was made that these bodies were the probable cause of 

 the developmental slow-down that initiated twinning. 

 Since, however, the protozoologist in question subse- 

 quently withdrew his original diagnosis of the supposed 

 intracellular parasite, this suggested cause of the lowered 

 rate of metabolism had to be abandoned. Up to this 

 time it was known that each set of quadruplets was the 

 product of a single egg, but the exact time and mode 

 of twinning was not definitely known. 



We are indebted to Patterson (19 13) for giving us a 

 detailed account of the twinning process. He discovered 

 in considerable numbers blastocysts in pre-twinning 

 stages and also found many stages of twinning. First 

 the originally single ectodermic vesicle elongates in the 

 bilateral axis of the uterus and twin thickenings of the 

 apical ectoderm are formed. This is a true twin stage. 

 Then each of the twins divides bilaterally into two 

 embryos, making two pairs of twins, or a set of quadru- 



