CAUSES OF TWINNING IN ARMADILLOS 107 



of the ectodermic vesicle; (/) the physiological isolation 

 of two, then four, growing regions on the blastoderm; 

 (g) the independent development from the four growing- 

 points of four complete embryos. 



Almost equally important for an understanding of 

 our problem is the very obvious renewal of developmental 

 activity as soon as placentation is attained. The embryo 

 is first aroused only locally, in the region of the ''Trager, " 

 but, with the renewed vigor acquired through the 

 establishment of this first placental connection, the wave 

 of renewed growth energy sweeps dis tally through the 

 tissues and ultimately strikes the ectodermic vesicle. 

 This region of the embryo has been the most profoundly 

 inhibited, partly owing to its inclosed position and partly 

 because the ectoderm, and particularly the nervous 

 ectoderm, is the most susceptible region of the embryo 

 to growth-depressing agencies. It is this vesicle in 

 which we get our first visible evidences of twinning, 

 though it is my belief that the true physiological isolation 

 of the quadruplet primordia considerably antedates their 

 visible or morphological isolation. Once this isolation 

 is accomplished twinning has occurred. 



To summarize, then, there is a fairly general agree- 

 ment among those of us who are interested in the 

 causes of twinning that, in the armadillo, the funda- 

 mental cause is interrupted or retarded development at 

 a critical period, followed by an isolation of four 

 growing-points. The exact mechanism of isolating the 

 four growing-points is still undetermined. Two theories 

 prevail at present: the budding theory of Patterson 

 and of Stockard, and the fission theory of which I am 

 an advocate. 



