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 nri nn. 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 hi 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 hi 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 history 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. 



