526 THE AMEBIC AN NATURALIST [Vol. XL VII 



placenta or Trager arises. Up to this time there is no 

 sign of polyembryony. The first step in the direction of 

 a division of the single embryonic vesicle into four em- 

 bryonic rudiments is seen in connection with mesoderm 

 formation. The mesoderm arises at two points, to wit 

 the extreme right and left sides of the laterally elongated 

 vesicle, and soon assumes the form of two hollow pouches 

 that subsequently expand and fuse together in the median 

 lines into a common extraembryonic body cavity. This 

 mode of origin of the mesoderm shows that the embryo 

 is no longer developing as a unit, but that there has 

 arisen a bilateral duality of function, due probably to the 

 partial physiological isolation of the right and left sides 

 of the mesoderm. The possible cause of this isolation 

 will be discussed presently. The first recognizable rudi- 

 ments of the embryos appear as two blunt processes or 



