GERM CELLS IN TATUSIA NOVEMCINCTA 345 



It will be recalled that in his paper on ''Polyembryonic de- 

 velopment in Tatusia novemcincta," Patterson ('13) has given 

 a thorough treatment of the early development of the armadillo. 

 For those, however, who are not sufficiently familiar with the 

 facts to follow the migration of cells, a brief resume may be 

 profitable. After the usual cleavage stages and the formation 

 of a typical mammalian blastocyst, consisting of one tropho- 

 blastic layer and an inner cell mass of embryonic cells, a process 

 of differentiation sets in through the migration of entodermal 

 mother cells from among the ectodermal cells. These cells, 

 directly or after division migrating to the under surface of the 

 cell mass, presently become transformed into a continuous layer 

 which splits from the ectoderm. Following this, the embryonic 

 ectoderm rounds up into a spherical mass which withdraws from 

 the trophoblast, and pushes into the vesicle cavity, becoming 

 included in a layer of entoderm. Through a process of vacuoliza- 

 tion, the ectoderm sphere now becomes a vesicle. It is after this 

 stage that the primary buds first appear from thickened areas 

 which have arisen on opposite sides of the ectodermic vesicle 

 through a shifting of cells. The primary buds show no signs of 

 embryonic primordia, but each directly gives rise to two second- 

 ary diverticula, thus forming four buds which soon extend and 

 begin to show the beginnings of four primitive streaks destined 

 to give rise to the quadruplets. Each embryo derives its ecto- 

 derm from a portion of the lateral plate, while the entoderm arises 

 in loco from the primitive entodermal sac. This description 

 though incomplete is sufficient to present the main points of in- 

 terest, and to show that a common point of origin for germ cells 

 of the 4-embryos of a blastocyst might, under the conditions, 

 be considered probable. 



STRUCTURE OF GERM CELLS 



Before proceeding to the history of the germ cells in the Arma- 

 dillo, a description of the form and structure of the cells under 

 consideration might be of value. On the whole, the form of the 

 germ cells is almost constant from the earliest stages up till the 

 time of the indifferent gonad. The size is equally constant^at 



