CAUSES OF TWINNING IN ARMADILLOS iii 



might be merely an expression of a bilateral arrangement of meso- 

 derm similar to that of many other vertebrate embryos [italics mine], 

 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 directed toward the openings of 

 the Fallopian tubes. 



This important statement seems to me to settle at 

 least two questions that have puzzled us for some time. 

 The first is that we have revealed to us the origin of 

 the exact correspondence existing between the two pairs 

 of twins and the two halves of the uterus. It will be 

 recalled that there are always two fetuses attached to a 

 right-hand placental disk and two more to the left-hand 

 disk. These are natural twin pairs and show many 

 evidences of an extremely close relationship. It is al- 

 most impossible to conceive of the vesicle as a bilateral 

 organism coming to lie in such a fashion as to have its 

 plane of symmetry coincide with that of the uterus. 

 The alternative view, and one that agrees well with the 

 facts, is that, either after its long period of quiescence 

 the vesicle has lost any bilaterality that it may have 

 possessed, or that up to that time it had never developed 

 an axis of symmetry. Only the axis of polarity had 

 been established and this had been nearly obliterated. 

 The crosslike placental area of the uterus (see The 

 Biology of Twins, p. 31) is very precise in its topographic 

 outlines and it seems clear that the vesicle soon comes 

 to be influenced by its location in such a way that a new 

 symmetry system arises in response to the symmetrical 

 conditions of the uterine environment. That the envi- 

 ronment does, in certain cases at least, determine the 

 axial and symmetrical relations of developing organisms 

 has been repeatedly demonstrated by various authors, and 



