Ii8 THE BIOLOGY OF TWINS 



the zygotically determined sex remains unaltered, for a 

 zygotic female will produce female-differentiating hor- 

 mones and will produce an adult female individual. 



IS TWINNING HEREDITARY? 



In the polyembryonic armadillos (Dasypus novem- 

 cinctus and D. hyhridus) it goes without saying that 

 their peculiar process of twinning is hereditary ; it is the 

 only mode of reproduction in these species. What is 

 really inherited in these cases is not fully understood, 

 but it is believed that its basis lies in some physiological 

 peculiarity of the egg, which causes it to have an abnor- 

 mally slow early development. As brought out in the 

 earlier chapter, this retardation in the developmental 

 rhythm produces an early fission of the embryonic 

 materials into several distinct primordia each of which 

 produces an embryo. Whatever the cause of poly- 

 embryony, it is unquestionably a specific character and 

 therefore hereditary. 



In the armadillo Euphractus villosus, and possibly 

 in other twinning species, the production of dizygotic 

 twins is practically a specific character and is therefore 

 inherited. The inherited character is some physiological 

 peculiarity of the ovarian rhythm resulting in the 

 synchronous ovulation of an egg from each ovary. In 

 close relatives of this species the ovaries alternate in 

 functioning, so that the right ovary would function in 

 one pregnancy and the left in the next. The breaking 

 up of this alternating rhythm in Euphractus and the 

 establishment of a synchronic rhythm in the two 

 ovaries would appear to involve a profound alteration 

 of the general metabolism. That this altered condition 



