26 THE BIOLOGY OF TWINS 



Further investigations concerning the mechanics of 

 this unique type of reproduction were forestalled by the 

 curiously misleading investigation of Rosner,' who 

 in 1901 published an account of a microscopic study of 

 the ovaries of a single specimen of D. novemcinctus 

 sent him by von Jhering. Rosner found that many 

 of the ripe follicles contained several eggs and that 

 the two largest follicles each contained four eggs, a 

 number corresponding to that of the fetuses of a litter. 

 On the basis of these observations he decided that 

 von Jhering's theory of the origin of the set of embryos 

 from a single fertilized egg was incorrect, and that the 

 four embryos typical for a Htter in D. novemcinctus were 

 derived from a nest of four eggs inclosed in a single 

 foUicle. No attempt was made to account for the 

 fact that all in a htter were of the same sex. Unfor- 

 tunately Rosner happened to examine a pair of patho- 

 logical ovaries, as has been abundantly shown through 

 my own studies and confirmed by the independent 

 observations of several other investigators. It is very 

 unusual to find more than a single egg in a follicle; in 

 fact, only one pair of ovaries out of nearly thirty that 

 have been sectioned for my studies shows any but 

 normal monovic follicles. 'Since, as Rosner erroneously 

 claimed, the four quadruplets came from four fused ova, 

 the problem appeared to be solved. On this account, 

 perhaps, no special interest was taken in the armadillo 

 situation for some time. 



In the year 1909 the question was reopened, when 

 almost simultaneously there appeared preliminary ac- 

 counts of the conditions in two species of Dasyptis. 



' M. A. Rosner, Bull. Acad. Sci. de Cracovie, 1901. 



