INHERITANCE OF VARIATES IN THE ARMADILLO 151 



During a sojourn of three weeks in the armadillo country I was 

 able to secure 158 new sets of quadruplets, together with the 

 preserved armor of the mothers. Of these 137 sets are sufficiently 

 advanced to admit of an accurate examination and enumeration 

 of the scutes of the banded region. A somewhat smaller number 

 are available for a study of the tail characters and the armor of 

 the cephalic plate, while only about twenty of the most advanced 

 sets are available for work upon the scapular and pelvic shields, 

 ill which the scutes are smaller and less well defined than else- 

 where. 



We have now a somewhat more adequate embryonic back- 

 ground for the study of heredity than was available when the 

 former paper was written. Patterson, continuing the study of 

 the early development of the species begun in collaboration with 

 the present writer, has recently traced the origin of the four fetuses 

 from a previously single blastodermic vesicle. He finds that, soon 

 after the completion of the process of germ-layer-inversion, the 

 inner ectodermic vesicle thickens up in two regions corresponding 

 to the right and left sides of the uterus and from these thickenings 

 grow out two buds of tissue, each of which subsequently under- 

 goes a dichotomous splitting in order to form the primordium of a 

 pair of embryos, each pair being the product of an originally 

 single bud. Genetically the individuals of a pair are more closely 

 related than are those of opposite pairs, and the closer resemblance 

 between individuals of a pair is simply explained. It was shown 

 in our earlier paper (Newman and Patterson '10) that at the prim- 

 itive streak stage the materials destined to form each of the four 

 embryos is isolated from those of the others, and that, when the 

 circulation is established, both the somatic and placental blood 

 supply of each fetus is entirely separate. It is still impossible to 

 state definitely the exact time at which the physiological isolation 

 of the individual embryonic primordia occurs, but we are safe in 

 assuming that there could be no shifting of the ectodermal ele- 

 ments from one hemisphere to the other after the end of the period 

 of germ-layer-inversion, when the whole ectoderm is a minute 

 body containing perhaps a hundred or so cells. Nature, therefore, 



