TWINNING IN DASYPUS NOVEMCINCTUS 53 



That region of the original trophoblast which is called 

 diplotrophoblast and lies at the distal pole of the vesicle 

 and within the ring formed by the fusion of the endo- 

 derm and the trophoblast proper has now entirely dis- 

 appeared and the distal portion of the vesicle is bounded 

 externally by endoderm. The point of fusion between 

 the endodermic and trophoblastic parts of the vesicle 

 wall is not easy to detect, but the endoderm is likely to 

 be more deeply stained in microscopic preparations. 



As the embryonic ectoderm grows down the sides 

 of the egg toward the Trager it carries with it the 

 adjacent endoderm, so that subsequently a large pro- 

 portion of the vesicle comes to be covered with 

 endoderm.^ 



Stage IX. The attachment of the quadruplet embryos 

 to the Trager (Fig. 17). — The figure is redrawn from one 

 published in 1910.^ The aim has been to represent the 

 vesicle as a transparent object, and this has been, 

 partially at least, realized. In the previously published 

 figure the axis of the vesicle was incorrectly placed so 

 that the heads of the embryos were directed toward 

 the top of the page. In all previous figures the original 

 animal pole of the egg is toward the top of the page, and 

 in order to preserve this arrangement with consistency, 

 all figures must show the Trager end of the egg at the 



^ Reference to Fig. 27, which is taken from one of Fernandez' 

 figures illustrating conditions in the Mulita (D. hybrid us), will serve 

 to show the' extent of the peripheral endoderm. It also makes clear 

 the relations of the germ-layer components of the entire vesicle. This 

 figure serves equally well for our species, D. novcmcinctus. 



^H. H. Newman and J. T. Patterson, "Development of the Nine- 

 banded Armadillo from the Primitive Streak Stage to B'lxih,'' Journal 

 of Morphology, V, 21. 



