S6 THE PHYSIOLOGY OF TWINNING 



taken by Gemmill, and if true would apparently account 

 for the partial uniting of two embryonic axes into one 

 in the case of conjoined twins. Since the germ ring is 

 supposed to represent the two sides of the axis, what 

 would happen if a second head or embryonic shield arose 

 to the right or to the left of the original shield ? Obvi- 

 ously the two heads would be in competition for that 

 region of the germ ring between them. The germ ring 

 is gradually taken up partly by one embryo and partly 

 by the other. They would therefore have separate 

 inner sides as long as the common region of the germ 

 ring lasted, but they would sooner or later use this up 

 and neither embryo would have any more material for 

 its inner half. Beyond this point, therefore, the two 

 outer halves of the germ ring, one half belonging to one 

 embryo and the other to the other, would concresce in 

 the median line to form the single or untwinned part of 

 the body. Thus we would readily obtain embryos with 

 two anterior regions and a single or common posterior 

 region. The farther apart the embryonic shields arise 

 on the germ ring the greater the extent of the twinned 

 or double region. If embryonic shields arise on opposite 

 sides of the germ ring the twins would be entirely 

 separate, except that they have a common yolk stalk, 

 a condition that could hardly be avoided since the two 

 embryos must have a common blastopore and are on a 

 single yolk sac. This all sounds reasonable enough on 

 the basis of the concrescence theory, but unfortunately 

 this theory is now practically discredited. No longer 

 can it be maintained that the germ ring represents the 

 separated right and left halves of the embryonic axis, 

 for it has been proved experimentally by several reliable 



