48 THE BIOLOGY OF TWINS 



originally situated at or near the animal pole of the 

 egg, has progressively retreated from this pole and now 

 lies with its apical part (head end) almost at the oppo- 

 site (vegetative) pole. The formerly voluminous cavity, 

 shown in stages I-IV, which we have called the tropho- 

 blast cavity, has gradually diminished in relative and 

 absolute volume (stages V and VI) until it is merely 

 a fiat crevice (ys) between the endoderm and the 

 trophoblast. At this stage the free edges of the endo- 

 derm have fused with the trophoblast at r, and that 

 portion of the trophoblast distal to the ring of fusion 

 (the diplotrophoblast) has thinned out preparatory to a 

 subsequent total disappearance, as in stage VII. The 

 retreat of the ectodermic vesicle from pole to pole and 

 the crowding out of the original trophoblast cavity 

 appear to be due to the pressure of the rapidly enlar- 

 ging extra-embryonic cavity (ex c), which is now lined 

 internally with a complete vesicle of mesoderm (ms). 

 That part of the mesoderm next to the ectodermal 

 amnion completes the amnion proper. No embryonic 

 mesoderm is as yet formed. 



The ectodermic vesicle is seen to be flattened against 

 the endoderm, and two hollow evaginations are shown 

 at right and left sides; these are the primordia of the 

 primary embryos (II and IV). These outgrowths con- 

 stitute twin embryonic areas with the apex or head 

 end of each pointing toward the apex of the ectodermic 

 vesicle (X), and with the posterior or growing end of 

 each pointing the one toward the right and the other 

 toward the left side of the uterus. It seems quite 

 evident that the bilaterahty of this twin embryonic 

 vesicle has been secondarily imposed upon it by the 



