406 H. H. Newman and J. T. Patterson. 



the individuals in a litter of rats or other mammals cannot at 

 present be determined. 



Although the difference between the two pairs of a litter may 

 on the average be rather marked, that between the individuals 

 of a pair seldom exceeds three scutes and averages in all cases 

 observed less than three, while cases of absolute identity in the 

 total number of scutes is of frequent occurrence. 



It will be remembered also that in our discussion of pairing a 

 considerable mass of evidence was adduced to show that even in 

 atypical scute arrangements a high degree of identity existed 

 between pairs, while in most cases the pairs differed greatly from 

 each other. All of these observations go to show that the identity 

 between the individuals of a pair is a very real thing but that the 

 there is nothing approaching true identity' between the pairs. 

 The condition may well be described as a case of double identical 

 twins. 



XI. Specific Polyembeyony and the Determination 



OF Sex 



The first clue to the existence of polyembryony in the armadillos 

 was furnished by the discovery that all of the individuals of a litter 

 are of the same sex. This together with his observation of a com- 

 mon chorion, led von Jhering to surmise that all of the embryos 

 of a vesicle arise from a single fertilized egg. That this flash 

 of insight foreshadowed the discovery of a truth has been suffi- 

 ciently demonstrated, we believe, by Fernandez for Tatu hybri- 

 dum and by us for T. novemcinctum. 



Identity of sex then is in some way closely bound up with the 

 phenomenon of polyembryony. Presumably all of the individuals 

 of a litter are of the same sex because they have been derived 

 from a single fertilized ovum ; but this presumption involves the 

 corollary that sex is determined in the germ before any demarka- 

 tion of embryonic rudiments has occurred. The only alternative 

 is that similarity of environmental conditions during the devel- 

 opmental period has the effect of producing offspring all of the 

 same sex, an alternative with no factual basis, as is shown by the 



