POLYEMBRYONIC DEVELOPMENT IN TATUSIA 593 
The cavity has been previously termed the ‘common amniotic 
cavity’ (Fernandez ’09, and Newman and Patterson 710), because 
it is common to the amniotic connections of the four embryos 
which later take their origin from the ectodermal vesicle. 
It is evident from this that in the mode of amnion formation the 
armadillo is to be classed with that group of mammals in which 
the amniotic cavity is from the first an enclosed space, and never 
has a free communication with the space outside the trophoblast. 
The cavity is always intra-trophoblastic (Hubrecht ’08), and 
consequently no folds ever arise to deliminate it, for this is not 
necessary. The armadillo can therefore be added to Hubrecht’s 
(08) compiled list of mammals, in which the amniotic cavity arises 
within the ectoderm and remains a closed vesicle. His list is as 
follows: ‘‘Cavia and other rodents, Pteropus, Galeopithecus, 
Erinaceus, Gymnura, monkeys and man.5 
ORIGIN OF THE EXTRAEMBRYONIC MESODERM OR 
MESOTHELIUM 
The origin of the mesoderm in mammals is a problem which has 
given rise to much difference of opinion among embryologists, but 
it is not necessary here to enter into this controversy. The first 
appearance of the mesoderm in the armadillo blastocyst is seen in 
connection with the cavity which lies between the ectodermal 
vesicle and the placenta, or what has been termed above, the extra- 
embryonic cavity. 
The entoderm, at the angle where it parts company with the 
ectodermal sphere to join the trophoblast (fig. 51), gives rise to a 
few cells which frequently become scattered throughout the extra- 
embryonic cavity. There is good reason for believing that most 
of these cells undergo disintegration pari passu with the develop- 
ment of the ectodermal vesicle. At any rate there comes a time 
when the exocoelomic space is essentially free from such ento- 
dermal cells (fig. 54). In some of the sections of this series a few 
of these cells are still found within the cavity. Thus in figure 55 
there are four of them, two lying some distance below the ectoderm 
and slightly to the left of the center, and two situated against the 
by Loc: ert; p:. 71. 
