POLYEMBRYOXIC DEVELOPMENT IN TATUSIA 627 



These citations will suffice to indicate how widespread is the 

 occurrence of polyovular follicles among mafrimals; but, although 

 they have a distribution among widely separated forms, their 

 occurrence in a given species seems to be rare. Thus O'Donoghue 

 ('12) found them but twice in forty-five individuals of Dasyurus; 

 and Schron ('63) only twice hi the ovaries of four hundred cats 

 and but once in eighty dogs. The writer has examined in all 

 probably more than fifty pairs of ovaries from the Texas arma- 

 dillo without finding a single case. 



In the light of these data it is impossible to associate the occur- 

 rence of polyovular follicles with the causation of polyembryonic 

 development in mammals. The fact that Rosner has found a 

 single armadillo with such follicles can have no greater impor- 

 tance than have the similar sporadic cases in certain other mam- 

 mals. The significant fact is, rather, that hi the ovaries of fifty 

 individuals belonging to a species which reproduces by specific 

 polyembryony alone not a single case of polyovular follicles 

 was found. 



It is well to emphasize the fact that the polyovular follicles do 

 not lie at the basis of polyembryony, for to accept Rosner's 

 theory would be equivalent to a denial altogether of the phenom- 

 enon of poh r embryonic development hi the Mammalia. A 

 multiple gestation from ova which have accidentally become 

 associated together during a part of the ovarian history, through 

 the fusion of .adjacent follicles, may have no more interest or 

 significance than a similar gestation resulting from ova from 

 uniovular follicles, but simultaneously ovulated, as occurs in 

 many mammals that are normally multiparous. One cannot 

 hope to throw much light on such fundamental biological problems 

 as those of sex determination, the limits of hereditary control, 

 and others, by the study of this type of development. It is only 

 to those cases in which the several embryos of a multiple preg- 

 nancy have taken their origin from a single fertilized egg that we 

 must look for facts with which to elucidate these problems. 



It is not intended here to underestimate the biological impor- 

 tance of polyovular follicles and of multiple gestations other 

 than those of polyembryony; but rather to point out that these 



