Development of the Nine-Banded Armadillo. 399 



On the whole however, in spite of these exceptions, the general 

 rule holds good, that the closest resemblances occurs between 

 paired embryos. 



In this connection it should be mentioned that even where 

 there is exact resemblance between the individuals of a pair in 

 the total number of scutes in the nine bands of armor, there is 

 no perfect correspondence with respect to individual rows. The 

 resemblance in total numbers of scutes is however, a matter of 

 more importance than the exact manner of their arrangement 

 into rows, which is a secondary process. Each primary scute 

 is the equivalent of a well defined hair group atid these groups, 

 as can be seen in other regions of the body, are quite definite units, 

 although subject to more or less shifting before reaching their 

 final arrangement into rows. In a subsequent paper we expect to 

 make a special study of variation and heredity in the elements 

 of the armor and shall in this place refrain from any more detailed 

 reference to the subject. 



Another source of data, however, which furnishes striking evi- 

 dence of pairing is seen in connection with a fairly common ten- 

 dency for regional fusion of adjacent bands of armor, or for the 

 occurrence of interrupted and of incomplete bands in definite 

 regions. Such atypical conditions occur in from three to four 

 per cent of all cases, a fact that we have established from an exam- 

 ination of considerably over a thousand shells. This comparative 

 rarity of occurrence, while it renders the collection of data on 

 pairing and identity difficult, gives to such data an added value, 

 in that chance resemblances are very unlikely to occur. 



Only four cases of strikingly atypical armor arrangements have 

 so far been discovered in the collection of foetuses now in our 

 possession'. In one case in embryos I and II there occurred a 

 remarkably atypical scute arrangement in the first band of 

 armor, while III and IV were quite normal. In a second case 

 I and II showed a slight fusion between the first two rows at the 

 right hand margin, while III and IV showed a much more esten- 

 sive fusion in exactly the same region. The pairing in this case 

 was only a matter of degree of fusion, but there was a decided differ- 

 ence in extent of the region of fusion in the two pairs. In a third 



