I04 THE PHYSIOLOGY OF TWINNING 



certain rather intangible morphological conceptions, such 

 as "discontinuous mode of development," ''develop- 

 mental moments," ''developmental arrests," and "alter- 

 nation of generations." Stockard accepts my view 

 that twinning in the armadillo and elsewhere is a result 

 of an early retardation of development or "arrested de- 

 velopment." He also adopts the budding hypothesis of 

 Patterson, especially the latter's view that one embryo 

 of each pair is a sort of accessory bud given off laterally 

 from, the primary bud. 



Stockard credits me with having appreciated the 

 significance of the "period of quiescence" in twin 

 formation, but claims that I overlooked what he himself 

 considers the very important fact that, during this 

 period, the blastocyst lies free in the uterus. This, in 

 spite of the fact that two references to this very fact 

 were made on page 39 of my Biology of Twins. That 

 there was no failure on my part to appreciate the physio- 

 logical significance of the belated placentation is evi- 

 denced by the statement on page 88 to the effect that it 

 is only after the physiological isolation of the quadruplet 

 primordia that a nutritive connection is established 

 between the embryonic vesicles and the maternal tissues. 

 It seemed to me at that time almost an obvious inference 

 from the facts that the "period of quiescence" and the 

 failure to undergo placentation were physiologically 

 related, but no categorical statement to that effect was 

 made. 



Stockard, therefore, is to be credited with emphasizing 

 this point. Undoubtedly failure to undergo placentation 

 at the time when eutherian mammals usually do is the 

 immediate cause of the arrest of development. Evi- 



