I02 THE PHYSIOLOGY OF TWINNING 



plets. Once these four twin primordia are established, 

 each develops its own amnion, allantois, and placenta; 

 and they remain essentially isolated, though surrounded 

 by a common chorion, till birth. 



In his search for still earlier embryonic stages (the 

 late and early cleavage stages) Patterson made a very 

 important observation, the significance of which he 

 failed entirely to appreciate. He began collecting 

 earlier and earlier in the season for several successive 

 years and found no earlier stages, but did find abundant 

 instances of single untwinned vesicles lying free in the 

 uterus. A cytological study of these vesicles showed 

 that they were not developing, since no mitotic figures 

 were to be found in any of the tissues. This ''period of 

 quiescence" lasted at least three weeks, and probably 

 longer. 



Here then was unequivocal support of my original 

 theory that twinning was due to a developmental slow- 

 down, and I immediately realized the importance of 

 this, but it was not until 191 7 that a further elabora- 

 tion of my theory of twinning was made public. In the 

 volume on The Biology of Twins the significance of the 

 ''period of quiescence" described by Patterson was 

 discussed, and twinning was explained as the direct 

 result of this period of quiescence. The view then 

 expressed was that, as the result of a very marked 

 retardation in the rate of development, the original 

 apical region lost its dominance over subordinate regions 

 and that, when placentation occurred and development 

 was resumed, at first two new centers of growth or apical 

 points arose, and later two others became isolated; so 

 that, instead of one apical end or head, four head pri- 



