Development of the Nine-Banded Armadillo. 405 



X. The Question op Identity of Embryos 



In the case of identical or monochorial twins the question of 

 close resemblance has been much discussed and the impression 

 seems to prevail that the individuals of a pair show such marked 

 similarity in their finer details of structure as to be practically 

 identical. 



In our earlier contribution to this subject we were inclined to 

 look for the resemblances between the embryos of a litter and to 

 understimate the value of the points of difference. Now however 

 that we feel that the question of specific polyembryony has been 

 established, the differences among embryos interest us more 

 than the resemblances, because they indicate a rather marked 

 degree of versatility in the hereditary possibilities of a single 

 fertilized germ cell. 



The only point of unfailing identity among the individuals of 

 a litter is that of sex. In 38 cases where the sex was definitely 

 determined there was no exception to the rule that all embryos 

 in a vesicle are of the same sex. 



So far as dimensional differences go there is again practical iden- 

 tity, although in a few cases there seemed to be a slight difference 

 in the size of the two pairs. In comparing one individual with 

 another we were forced to admit that they differed only in the 

 minutest details, such as the number of scutes in the armor. A 

 comparison on this basis is just about as searching as would be a 

 comparison of the number of feathers in a given feather tract of 

 two birds, or of the hairs in a given hair area of two mammals. 

 We have for the present limited our comparison to the total num- 

 ber of large scutes (with corresponding underlying bony plates), 

 in the nine moveable bands of armor. The extreme range of 

 variability in the total number of these plates (in all of the indi- 

 viduals so far examined) is rather wide, running from 511 to 620, 

 a range of 109. In a number of cases the individuals of a litter 

 exhibit a range of only five or six scutes, but as a rule the range 

 is wider, averaging in all cases studied twelve, or less than one- 

 ninth of the total range of our sample of the species. Whether 

 or not this represents a closer esemblance than exists between 



