ii8 THE PHYSIOLOGY OF TWINNING 



of budding. Patterson has noted that buds II and 

 IV seem to occupy the original site of the "primary 

 buds," that is, respectively toward the right and toward 

 the left, while buds I and III appear to come off slightly 

 behind and to the left of, respectively, buds II and IV. 

 He is therefore inclined to consider buds II and IV as 

 continuations of the original twin primordia, the 

 ''primary buds," and that I and III are lateral or 

 accessory buds. In support of this view it is not infre- 

 quently noted that I and III are usually not quite so 

 advanced as are II and IV. The lateral budding notion 

 can hardly continue to be valid in view of what has 

 already been stated, but it is true that the position of 

 one of each pair remains lateral and the other Kes to its 

 left. There is evidently some very accurately balanced 

 behavior here, whatever its significance. The result 

 is that each embryo comes to occupy its full quadrant 

 of the vesicle. What appears to happen is that one of 

 the migrating embryos keeps the original direction while 

 the other is shunted off at a considerable angle. Why 

 in both cases the shunted-off individual goes to the 

 left is a question that at present I am entirely unable to 

 explain. A careful re-examination of my own mate- 

 rial and of some of Patterson's figures leaves it an 

 open question in my mind whether the right embryo 

 of each pair always retains the original direction of 

 growth and the left is caused to diverge. There are 

 certainly some cases in my possession where the point 

 of attachment of the placenta is more nearly lateral 

 in the left-hand embryo than in the right and there are 

 undoubtedly many instances in which the left-hand 

 embryo of a pair is more advanced in development than 



