CAUSES OF TWINNING IN ARMADILLOS 113 



is anticipated soon after this period by certain easily detectable 

 changes in the walls of the vesicle. It will be recalled that immedi- 

 ately after the ectodermal sphere has become transformed into 

 a vesicle, that portion of the wall of the vesicle which is turned 

 toward the free pole of the blastocyst is of a relatively uniform 

 thickness. Very shortly thereafter one can detect a tendency in 

 this region of the wall to become less thick. The thinning out 

 may be due in part to an increase in size of the vesicle by the 

 accumulation of fluid within its cavity, but undoubtedly in the 

 main it is brought about through the shifting of cells from here to 

 the lateral portions of the wall, for these show an increase in thick- 

 ness. 



The shifting of cells from the pole of the vesicle results in 

 the formation of a thickened zone adjoining the thin or endothelial- 

 like portion of the ectodermal vesicle. The zone is not uniformly 

 thick, but is thickest at the two regions corresponding respectively 

 to the right and left sides of the vesicle. One can therefore 

 correctly speak of these thickened areas as lateral plates. 



The primary buds arise from these lateral plates, and appear 

 as two broad, blunt processes protruding from the sides of the 

 ectodermal vesicle. Each bud involves the greater portion of the 

 side of the vesicle, covering an arc of approximately 80 degrees 

 on the circumference [Fig. 47, p. 114]. 



What better description of a fission process could one 

 ask for than this? The pre-twinning stage of the 

 vesicle, which it must be remembered has undergone 

 germ-layer inversion so that the cells originally apical 

 in position are now occupying the distal pole of the 

 ectodermic vesicle, is characterized by the fact that the 

 polar area is thickest. This was the prospective locus 

 of the head of the untwinned embryo. Then this apical 

 area becomes relatively thinner down the middle and 

 two bilaterally arranged, thickened areas arise and are 

 distinctly separated by the median, thinned-out area. 

 Is this budding or fission ? The so-called primary buds 



