TWINNING IN DASYPUS NOVEMCINCTUS 51 



-dermic vesicle had already been physiologically differ- 

 entiated into a number of radially arranged equivalent 

 apical points focusing toward the original common apex, 

 and that those particular apical points which hai)pened 

 to be directed respectively to the right and left sides of 

 the uterus had more room in which to grow and con- 

 sequently developed more rapidly than those located in 

 other sectors, and thus 

 became the primary 

 embryos. The less 

 favorably situated 

 points that grow less 

 rapidly are secondary 

 and tertiary in time, 

 but genetically they are 

 as independent and as 

 old as the primary 

 embryos. This expla- 

 nation of the curious 

 bilateral orientation of 

 the quadruple vesicle 

 in the uterus accords 

 with the facts of devel- 

 opment and of heredity 

 better than others 

 which have been previously offered, and removes, it 

 seems to me, much of the mystery involved in the 

 problems arising from a study of resemblances and of 

 symmetry relations among the quadruplets. 



Stage VIII. The retreat of the embryos from the 

 common amnion toward the original animal pole of the 

 egg (Fig. 16). — In order to bridge over the gap between 



Fig. 15. — Outline of lower pole of 

 stage like Fig, 14, showing the four 

 embryos. The dotted lines across 

 are lines of certain sections not shown 

 here. The four embryonic areas are 

 numbered I, II, III, and IV. (From 

 Patterson.) 



