f the eggs of
Strongylocentrotus purpuratus treated in this manner may develop into
twins. These twins may remain separate or grow partially together and
form double monsters, or heal together so completely that only slight or
even no imperfections indicate that the individual started its career
as a pair of twins. It is also possible to control the tendency of such
twins to grow together by a change in the constitution of the sea-water.
If we use as a twin-producing solution a mixture of sodium, magnesium
and potassium chlorides (in the proportion in which these salts exist in
the sea-water) the tendency of the twins to grow together is much
more pronounced than if we use simply a mixture of sodium chloride and
magnesium chloride.

The mechanism of the origin of twins, as the result of altering the
composition of the sea-water, is revealed by observation of the first
segmentation of the egg in these solutions. This cell-division is
modified in a way which leads to a separation of the first two cells.
If the egg is afterwards transferred back into normal sea-water, each
of these two cells develops into an independent embryo. Since normal
sea-water contains all three metals, sodium, calcium, and potassium, and
since it has besides an alkaline reaction, we perceive the reason why
twins are not normally produced from one egg. These experiments suggest
the possibility of a chemical cause for the origin of twins from one egg
or of double monstrosities in mammals. If, for some reason, the liquids
which surround the human egg a short time before and after the first
cell-division are slightly acid, and at the same time lacking in one
of the three important metals, the conditions for the separation of the
first two cells and the formation of identical twins are provided.

In conclusion it may be pointed out that the reverse result, namely,
the fusion of normally double organs, can also be brought about
experimentally through a change in the chemical constitution of the
sea-water. Stockard succeeded in causing the eyes of fish embryos
(Fundulus heteroclitus) to fuse into a single cyclopean eye through the
addition of magnesium chloride to the sea-water. When he added about 6
grams of magnesium chloride to 100 cubic centimetres of sea-water and
placed the fertilised eggs in the mixture, about 50 per cent of the
eggs gave rise to one-eyed embryos. "When the embryos were studied the
one-eyed condition was found to result from the u