598 J. T. PATTERSON 



comes lessened upon the formation in the armadillo of a distinct 

 Trager epithelium, with a potential cavity lying between it and 

 the invaded mucosa (fig. 23). 



Other comparisons might be drawn between the armadillo 

 blastocyst and those of other mammals, but this is not necessary. 

 Sufficient evidence has been presented, I believe, to establish the 

 fact that the early stages of the armadillo give a history corre- 

 sponding in its general outlines to the development of an egg which 

 in other forms produces but a single individual. The differences 

 noted are no greater than would be expected to exist between 

 mammals as widely separated as the armadillo and the bat or 

 mouse. 



ORIGIN OF PRIMARY BUDS 



In the late stages of development it has been demonstrated 

 (Newman and Patterson '10) that the four embryos of an arma- 

 dillo litter are arranged within the single chorion in two pairs, 

 which hold a very definite orientation with reference to the uter- 

 ine axes. Thus it was found that one embryo always occupied that 

 portion of the chorionic cavity lying adjacent to the dorsal wall of 

 the uterus, one holds a ventral position, and the other two lie to the 

 right and left sides, respectively. The heads of the embryos are 

 always directed toward the cervix end of the uterus, and conse- 

 quently point in a direction exactly opposite to that of the head 

 of the mother. It was further found that the ventral embryo 

 (I) is paired with the right-lateral (II) and that the dorsal (III) 

 and left-lateral (IV) embryos are members of the other pair. 6 

 This relation was apparent, not only from the very close heredi- 

 tary similarity existing between the two individuals of a pair, but 

 also from certain foetal connections, notably the union of the 

 amniotic canals of each pair in comparatively early stages. 



The question now arises as to how such a striking relationship 

 between the embryos has come into existance; and in seeking 



6 The terms 'right-lateral' and left-lateral' refer to the position of the embryos 

 within the blastocyst, and not to the right and left sides respectively of the uterus. 

 For example, the right-lateral embryo lies on the left side of the uterine cavity. 

 Throughout the paper I use the Roman numerals (I to IV) to designate the em- 

 bryos, so that the reader will find no difficulty in locating any embryo to which 

 reference is made. 



