CAUSES OF TWINNING IN ARMADILLOS 115 



thin out so markedly as when the first twinning process 

 occurred. The twinned embryonic areas, one on each 

 side of the vesicle, do not completely separate but 

 remain united by a relatively thick band of ectoderm 

 and it is the gradual severing of this connection, beginning 

 at the posterior end, that Patterson has mistakenly 

 interpreted as a secondary budding process. His own 

 account shows this: 



The formation of the secondary buds immediately follows 



the establishment of the primary diverticula Each primary 



bud gives rise to two second- 

 ary' buds, and consequently 

 there are four secondary 

 diverticula. Each secondary 

 bud carries the rudiment or 

 primordium of an embryo. 

 The first step leading to the 

 development of the secondary 

 diverticula consists in the 

 formation of two thickenings 

 in the wall of each primary 

 bud. One of these areas lies 

 at the tip of the bud, while 

 the other appears slightly to 

 the left (as viewed from 

 above) of the tip. The sec- 

 ondary buds then arise from 

 these areas as bhnd diver- 

 ticula, which extend down 

 along the inner surface of the 

 yolk-sac entoderm. 



Let us look for a mo- 

 ment at Patterson's fig- 

 ure illustrating the four 

 ''buds" (Fig. 48). The 



Fig. 48. — Outline polar view of an 

 armadillo quadruplet egg after the 

 completion of the process of twin- 

 ning. The four embr>'onic areas are 

 entirely separate; each has established 

 its own axis and is about to migrate 

 backward toward the placental region. 

 The head ends of the four embryos 

 point toward the center of the vesicle. 

 The posterior ends are beginning to 

 push outward and give rise to the so- 

 called "buds" of Patterson. (After 

 Patterson.) 



