TWINNING IN DASYPUS NOVEMCINCTUS 63 



Fernandez has described and figured a rare case in wHch 

 a single fetus occupied an egg alone. The placental 

 relations were decidedly irregular in that the villous 

 area was an incomplete irregular zone or band partially 

 encirchng the equator of the vesicle. It seems evident 

 that this condition is not due to a sporadic reversion 

 to a uniparous condition but to the precocious death 

 of several embryos. The zonary position of the placenta 

 seems to require this interpretation, for, if the condition 

 is due to a sporadic recurrence of a type of development 

 typical for uniparous species of armadillo, the placenta 

 should be discoid and at the proximal pole of the vesicle. 



In my own collection of over two hundred eggs 

 there occurred one case of twins which were born in 

 captivity, and whose placental relations were not 

 determined; two sets of triplets, in one of which unmis- 

 takable traces of a fourth embryonic rudiment appeared; 

 and three sets of quintuplets, in one of which an addi- 

 tional sixth degenerating embryonic rudiment was evi- 

 dent. In all of these cases there was a pronounced 

 lack of adjustment to the uterine axes. 



One may therefore conclude that the quadruplet 

 condition is practically specific for D. novemcinctus, 

 and that there is little evidence that progressive evolu- 

 tion will either augment or decrease the typical number. 



Variations in the relations of embryos. — In over 75 

 per cent of cases the paired arrangement of embryos and 

 their rather precise method of union with the right and 

 the left placental disks is like that described in con- 

 nection with stage IX, Fig. 17. When I first studied 

 the orientation of embryos with reference to the uterine 

 bilaterahty, I was much impressed with the regularity 



