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 im 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 
