I30 THE PHYSIOLOGY OF TWINNING 



formed or were forming during the twinning process. 

 The type of twinning described by Streeter takes place 

 at an early blastula stage and should be classed as a 

 case of fission of an early blastoderhi to form two sepa- 

 rate embryonic primordia. 



Just as I was revising the manuscript of this volume 

 there appeared two papers by Arey (1922a, 19226), de- 

 scribing certain early human one-egg twins, that throw 

 new light on our present problem. It is not uncommon 

 to find in human beings cases of tubal pregnancy. One 

 or more ova are fertilized in a Fallopian tube and because 

 of certain pathological conditions remain in the tube and 

 acquire a sort of makeshift placentation. Embryonic 

 development may proceed for months before fetal death 

 occurs. Arey has brought together, after a rigid exami- 

 nation of the Kterature, some sixty cases of human tubal 

 twins, about two-thirds of which are monochorial. In 

 view of the fact that uterine monochorial twins (probably 

 always one-egg twins) are only about one-fourth as 

 numerous as dichorial twins, it appears that tubal mono- 

 chorial twins are eight times as frequent as ''might be 

 expected if the tube were no more favorable than the 

 uterus as a seat for twin production." 



Arey himself describes two cases of monochorial twins 

 that are especially significant: 



The first of the two specimens consisted of a single chorionic 

 sac which contained twin embryos, each 12.3 mm. long. There is 

 a common yolk sac from which distinct yolk stalks arise near 



together and pass to their respective umbilical cords 



The second new twin specimen is in some respects more interest- 

 ing. Within a single chorion were twin embryos of 11.5 and 



