ORGANIC SYMMETRY IN ARMADILLO QUADRUPLETS. 2OI 



to obliterate the original symmetry relations of the undivided 

 embryo. Similarly, when each primary bud subdivides to form 

 the secondary buds that are the primordia of the definitive in- 

 dividuals, a certain residuum of the primary bud symmetry 

 system is carried over, manifesting itself in mirror-imaging be- 

 tween the twins derived from the same primary bud. But here 

 again a certain amount of regulation occurs so that a third system 

 of symmetry, the bilateral symmetry of each individual, tends to 

 obliterate former systems of symmetry. 



A further evidence of the existence of a primary system of 

 symmetry relations is seen in the mode of formation of the 

 secondary buds. Each primary bud gives off to its left a secon- 

 dary bud as though the whole vesicle had a growth whirl to the 

 left. It appears to be acting as a single system at the very time 

 when polyembryonic processes are most active. 



(b} The Closer Symmetry of Pairs and its Significance. One of 

 the most striking early discoveries resulting from a study of 

 armadillo quadruplets was that the four fetuses of a set are 

 paired so that one pair is fixed to the right-hand placental disc and 

 the other pair to the left-hand placental disc. These discs lie re- 

 spectively on the right- and left-hand sides of the uterus. The 

 pairs are more strikingly identical than are the quadruplets as a 

 whole and they more often exhibit interindividual mirror-imaging 

 than do the individuals of opposite sides of the vesicle. It at first 

 seemed probable that this condition was due to the "fact," carried 

 over from the literature on the origin of human duplicate twins, 

 that one pair was derived from one and the other from the second 

 blastomere of the two-cell stage. Now that we know more 

 about the embryonic history of the armadillo we are less inclined 

 to relate the symmetry of the vesicle to the symmetry of the 

 oosperm. 



It will be recalled that the early blastodermic vesicle, when it 

 becomes fixed to the mucosa of the fundus uteri, adheres by means 

 of a predetermined disc of cells called the "Trager." This disc 

 is evidently derived from the animal pole of the egg and hence 

 we have a mechanism for fixing the principal axis of the embryo 

 and causing it to coincide with that of the mother. The blasto- 

 dermic vesicle, even if it had a predetermined bilaterality, could 



