Ii6 THE PHYSIOLOGY OF TWINNING 



first thing of importance that I note is what Patterson has 

 apparently forgotten : that the apical or head ends of the 

 four embryos are all directed inward toward the center 

 of the vesicle. The heads are already widely separated 

 as ar-e also the embryonic areas. The *'buds" are 

 merely outpushings at the posterior ends of the embryos 

 involving largely extra-embryonic (amniotic) ectoderm. 

 It should be entirely obvious from Patterson's own 

 account that the essential acts of twinning are entirely 

 finished before this "budding" process begins. What 

 then are these so-called ''buds" ? 



This question may be readily answered after studying 

 the typical embryonic history of non-twinning armadillos, 

 for it is here that the clue to their interpretation exists. 

 According to Fernandez (19 14), who has studied the 

 development of the non-twinning species Euphractus 

 villosus, the medullary plate of the single embryo arises 

 from the distal thickening of the ectodermic vesicle, 

 just as it begins to do in our twinning species. Instead, 

 however, of remaining at the distal pole, farthest from 

 the placenta, the embryo grows backward out of the 

 amnion, which remains attached to the distal endoderm. 

 It leaves the amnion by means of a process, the "bud" 

 of Patterson. Not only does the posterior end grow 

 backward but the head as well moves along the inner 

 wall of the ectoderm carrying attached to it an amniotic 

 connecting canal that remains as a hollow string connect- 

 ing the amnion of the embryo to the rudiment of the 

 original amnion at the opposite pole. It is evident, 

 therefore, that Patterson's "secondary buds" are merely 

 the beginnings of amniotic outpouchings which are 

 destined to act as migration canals through which the 



