POLYEMBRYONIC DEVELOPMENT IN TATUSIA 637 
the group of armadillos. It must be stated frankly that both of 
these questions are wholly unanswered. But a few suggestions 
are given in the hope that they may point the way to their solu- 
tion. In the remaining part of this section we shall discuss the 
question of phylogeny as the other problem was discussed in the 
preceding section. 
So far as I am aware, the only armadillos in which specific poly- 
embryony is definitely known to occur are the North and South 
American varieties of T. novemcincta, and T. hybrida, which 
inhabit the southern part of the South American continent. 
Judging from the description of Fernandez (’09) on comparatively 
late stages of T. hybrida, the two species agree very closely in 
many details of development; but there are evidently important 
differences, one of which relates to the number of offspring in a 
litter. In T. novemcincta a litter consists typically of four indi- 
viduals, while the number in T. hybrida varies from seven to 
twelve, with a strong tendencey to produce eight or nine. 
In T. novemcincta, out of 219 pregnant females that have been 
studied carefully, 176 had embryonic vesicles far enough advanc- 
ed to permit a determination of the number of embryos, and of 
these all but four, or about 98 per cent, possessed four embryos 
each. Of the four exceptions, three had five-embryo sets and 
one a two-embryo set. 
There is some doubt in the latter case, since the embryos were 
born in the laboratory and therefore an opportunity to study them 
in utero was not offered. But all of the five-embryo sets were 
carefully studied and the relations of the different parts of the 
blastocyst were determined. In each case two members of the 
litter (either on the right or on the left side) showed the normal 
paired condition, while the other three presented an asym- 
metrical arrangement, one of the primary placental discs being 
about twice the usual size and bearing the umbilical cords of two 
embryos. 
This arrangement of the members of a five-embryo set is signi- 
ficant, in that it suggests that the two embryos arose in the nor- 
mal fashion from one of the primary buds, and that the three 
embryos of the opposite side have come from the other primary 
