CHAPTER VIII 



THE CAUSES OF TWINNING IN THE 

 ARMADILLOS 



In 1909 we (Newman and Patterson) first discovered 

 and studied " specific polyembryony " in the nine-banded 

 armadillo of Texas {Dasypus novemcinctus texanus). It 

 was found that this species habitually gives birth to a 

 litter of four offspring, that all members of any given 

 litter are of the same sex, and that the members of 

 any one litter are usually strikingly alike. In 19 10 we 

 published a more detailed study of the development 

 of this species in which all stages from the primitive- 

 streak stage to birth were studied. In 191 1 we pub- 

 lished a statistical study of variation and heredity in 

 armadillo quadruplets and showed that the members of 

 a litter are as closely similar to one another as are the 

 right and left sides of single individuals; they have a 

 coefhcient of correlation of .9+ as compared with that 

 of ordinary siblings, which is about .5. It was also 

 clearly shown that the quadruplets were arranged in 

 two pairs and that the two individuals of a pair were 

 more nearly identical than are individuals belonging 

 to opposite pairs. In 191 2 I studied the oogenesis 

 and ovulation of the armadillo and showed that only 

 one egg is given off at a breeding, for only one corpus 

 luteum was formed in the ovary. The egg was an 

 entirely typical mammahan egg. In 19 13 I formu- 

 lated the first theory ever published as to the causes 



