orphosis into
the adult stage and since in all the experiments made by the writer
the parthenogenetic plutei lived as long as the plutei produced from
fertilised eggs.

(c). ON THE PRODUCTION OF TWINS FROM ONE EGG THROUGH A CHANGE IN THE
CHEMICAL CONSTITUTION OF THE SEA-WATER.

The reader is probably familiar with the fact that there exist two
different types of human twins. In the one type the twins differ as much
as two children of the same parents born at different periods; they
may or may not have the same sex. In the second type the twins have
invariably the same sex and resemble each other most closely. Twins
of the latter type are produced from the same egg, while twins of the
former type are produced from two different eggs.

The experiments of Driesch and others have taught us that twins
originate from one egg in this manner, namely, that the first two cells
into which the egg divides after fertilisation become separated from
each other. This separation can be brought about by a change in the
chemical constitution of the sea-water. Herbst observed that if the
fertilised eggs of the sea-urchin are put into sea-water which is freed
from calcium, the cells into which the egg divides have a tendency
to fall apart. Driesch afterwards noticed that eggs of the sea-urchin
treated with sea-water which is free from lime have a tendency to give
rise to twins. The writer has recently found that twins can be produced
not only by the absence of lime, but also through the absence of sodium
or of potassium; in other words, through the absence of one or two of
the three important metals in the sea-water. There is, however, a second
condition, namely, that the solution used for the production of twins
must have a neutral or at least not an alkaline reaction.

The procedure for the production of twins in the sea-urchin egg consists
simply in this:--the eggs are fertilised as usual in normal sea-water
and then, after repeated washing in a neutral solution of sodium
chloride (of the concentration of the sea-water), are placed in a
neutral mixture of potassium chloride and calcium chloride, or of sodium
chloride and potassium chloride, or of sodium chloride and calcium
chloride, or of sodium chloride and magnesium chloride. The eggs must
remain in this solution until half an hour or an hour after they have
reached the two-cell stage. They are then transferred into normal
sea-water and allowed to develop. From 50 to 90 per cent o