POLYEMBRYONIC DEVELOPMENT IN TATUSIA 599 
answer to this question we are at once confronted with the much 
more important problem of the origin of the multiple embryos 
from the single fertilized egg. In the paper cited above the 
position was taken that the embryos belonging to a pair were 
probably derived from one of the blastomeres of the two-celled 
stage, and that each embryo could ‘therefore be looked upon as 
a lineal descendent of one of the blastomeres of the four-celled 
stage. In the present contribution the view is held that the 
four embryos do not owe their origin to a spontaneous blas- 
totomy, but rather that they are the product of a form of 
agamogenesis belonging to the general category of budding. 
It was considered then of the utmost importance to determine 
at just what point in the development of the armadillo blasto- 
cyst evidence of its quadruplicity first appeared. Consequently 
a sharp lookout was maintained in the study of all the early 
stages for signs of the first expression of polyembryony. The 
earliest observed evidence which could be interpreted as repre- 
senting the beginning of multiple embryos comes in the forma- 
tion of the mesothelium—not in the manner in which the ele- 
ments of this layer arise, for localized centers of proliferation 
were not found, but in the early formation of the two large 
mesodermal vesicles through the fusion of several smaller ones. 
The development of two mesodermal vesicles would not in it- 
self be so significant, as it might be merely an expression of a 
bilateral arrangement of mesoderm similar to that of many other | 
vertebrate embryos, were it not for the fact that they hold a 
position corresponding exactly to the two primary ectodermal 
buds; that is, they lie on the sides of the vesicle which are di- 
rected towards the openings of the fallopian tubes. However, 
it may be that these. two mesothelial vesicles have no general 
significance with reference to polyembryonic development, for it 
must be kept in mind that they have arisen by the fusion of 
numerous smaller vesicles, and later they in turn fuse to form a 
single vesicle. 
Whatever may be the significance of the position of the two 
mesodermal vesicles, certain it is that the first indisputable evi- 
dence of the differentiation of the four embryos from the blasto- 
