112 THE PHYSIOLOGY OF TWINNING 



that this is only another instance of a very general situ- 

 ation seems evident. Much of the difficulty formerly 

 associated with the fact that the S3anmetrical relations 

 of armadillo quadruplets coincide with those of the mother 

 thus disappears when we view the fetal symmetry as de- 

 termined de novo by that of the uterus. The second point 

 of importance that is brought out by Patterson's descrip- 

 tion inheres in the italicized words, for I believe that the 

 bilateral arrangement prior to twinning which he points 

 out is not at all the bilaterality of the untwinned embryo, 

 but merely the result of a physiological isolation of two 

 halves of the vesicle. It is the equivalent of what 

 happens in a starfish blastula when two invagination 

 areas become physiologically isolated so as to lie in 

 equivalent positions to each other, so that each faces 

 the other like a pair of mirror-images. The separation 

 of the two gastrulation areas is obviously a sort of 

 migration of cells toward the right and left of the uterus, 

 leaving a thinned-out region in the middle line. This is 

 more like a fission process than a budding process, for 

 of the two embryonic areas it would be impossible to say 

 which is the original individual and which is the bud. 

 The concept of budding implies that the original apical 

 point retains its identity and that the bud is a secondary, 

 more or less lateral, new growing-point that has escaped 

 from the dominance of the original or primary growing- 

 point and has asserted its own independence. This is 

 evidently Patterson's idea of budding, at least in so far 

 as the formation of secondary buds is concerned, as will 

 be made clear from the following quotation : 



The primary buds do not develop for some time after the 

 completion of the ectodermal vesicle, although their appearance 



