78 



THE BIOLOGY OF TWINS 



A considerable collection of 34 embryonic vesicles 

 was assembled, showing stages from the primitive streak 

 stage up to a stage of advancement similar to that 

 shown in Fig. 32. The net result of the analysis of 



Fig. 32. — Photograph (after Fernandez) of a double embryonic 

 vesicle of the armadillo Euphractiis {Dasypus) villosus, showing the two 

 fetuses inclosed in what appears to be a common chorion and separated 

 by an amniotic partition composed of the fused amnia of the twins. 

 These come from two eggs and may be or may not be of opposite sex. 



this material was that the twins proved not to be 

 derived from one egg (i.e., were not monozygotic) at 

 all. The false appearance of polyembryony was due 

 to the very intimate fusion of two originally quite 

 separate eggs, which were merely crowded so closely 



