THE BIOLOGY OF TWINS 161 



secondary growing-points experience, for some 

 reason or other, physiological isolation, and proceed 

 to develop in independence. It is known that in- 

 jections of butyric acid and some other reagents into 

 fish embryos may bring about a sort of dislocation 

 or partial dissolution of the germinal area, and 

 that this " blastolysis " results in monstrosities. 

 As butyric acid may arise in a mammal's body as 

 the results of deranged carbohydrate metabolism, 

 Werber has suggested a physiological theory of the 

 origin of certain kinds of monsters. Now it may be 

 that the slight isolation or insulation of four foci 

 in the germinal vesicle of the armadillo is a step 

 in the direction of blastolysis which has not, how- 

 ever, crossed the limits of the normal. It may also 

 be, as Mr. Newman suggests, that the development 

 of two offspring from one egg is " only a phase of 

 the much more general phenomenon of symmetrical 

 division." Thus the building up of the right and 

 left sides of a bilaterally symmetrical animal is 

 essentially a twinning process. It is a very interest- 

 ing fact that of twin-brothers one is sometimes 

 right-handed and the other left-handed. 



In the Nine-banded Armadillo quadruplets spring 

 from one egg, but it is a curious fact that in the 

 Hairy Armadillo (Euphractus) twins arise from 

 two eggs whose external birth-robes (chorionic 

 foetal membranes) fuse together secondarily. Need- 

 less to say, these twins may be of different sexes, 

 while the quadruplets of Dasypus are always of one 

 sex. Here, then, we have an instructive hint of 



