CAUSES OF TWINNING IN ARMADILLOS 109 



been resumed and it is high time that an embryonic axis 

 developed. There is, however, no clearly defined head 

 process or primitive streak. The ectodermic vesicle, 



trpl 



dtr 



Fig. 46. — Section of an armadillo embryo, cut at right angles to the 

 sagittal plane, showing the first evidences of the prospective twinning 

 process. A secondary bilaterality has begun to be imposed upon the 

 embryo by the bilaterality of the uterus. The point X is the original 

 apical end of the single embryo. The two sides of the ectodermic 

 vesicle {ec) are thicker than the point X and are the blastoderms of the 

 twins. Tra, Trager or primitive placenta; en, endoderm; Ir pi, tropho- 

 derm plate; ec am, ectodermal layer of amnion; am c, amniotic cavity; 

 ms, mesoderm; ys, yolk sac; ex c extra-embryonic cavity. (From 

 Newman after Patterson.) 



