POLYEMBRYONIC DEVELOPMENT IN TATUSIA 625 
The mass of Trager tissue constitutes the material upon which 
the further growth of the chorionic wall depends, and after the 
4 to 5 mm. stage is reached it rapidly extends upward, becomes 
thinner, and carries before it the cap of the yolk-sac entoderm, 
in the manner already described. It also forms the basis for the 
formation of the villi which appear upon this region of the cho- 
rionic wall. At first the villi have a general distribution over the 
surface of the modified Trager zone, but in blastocysts which have 
attained a diameter of 25 to 30 mm.., the long branched villi become 
restricted to four distinct areas or patches. These patches are 
situated just below the boundary line between the Trager and the 
yolk-sac, and each villous area lies opposite the point at which an 
umbilical cord arises from the inner surface of the chorionic wall. 
The four villous areas give rise to the four placental discs of later 
stages, and, corresponding to the position of the embryos, are ar- 
ranged into two pairs. In the final stages of gestation the two 
discs belonging to a pair become closely associated together, thus 
forming two large double-discoidal placenta, which occupy respec- 
tively the right and left sides of the chorion. 
In the formation of the placenta, as well as in the general devel- 
opment of the blastocyst of the armadillo, there are many 
opportunities for comparison with the developmental stages of 
other mammals, but such comparisons can be more safely drawn 
after we have had a chance to work out a detailed piston of 
placentation. 
GENERAL DISCUSSION 
1. Theories of polyembryony 
A great many different views have been expressed in explana- 
tion of polyembryonic development. Most of these are pure 
conjectures, and as such hold no place in any serious attempt at 
a scientific treatment of the subject. Exceptions are made here 
to those theories only which seem to hold a grain of truth and 
which have gained a certain number of adherents. 
a. Theory of polyovular follicles. An attempt has been made 
to account for polyembryony on the basis of polyovular follicles. 
