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 and 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 exten¬
sive fusion in exactly the same region. The pairing in this case
was only a matter of degreeof fusion, but there was a decided differ¬
ence in extent of the region of fusion in the two pairs. In a third