in the position
of the cell or the organ may give results which can be traced to a
change in the position of the two substances. This is very nicely
illustrated by the frog's egg, which has two layers of very viscous
protoplasm one of which is black and one white. The dark one occupies
normally the upper position in the egg and may therefore be assumed to
possess a smaller specific gravity than the white substance. When
the egg is turned with the white pole upwards a tendency of the white
protoplasm to flow down again manifests itself. It is, however, possible
to prevent or retard this rotation of the highly viscous protoplasm, by
compressing the eggs between horizontal glass plates. Such compression
experiments may lead to rather interesting results, as O. Schultze first
pointed out. Pflueger had already shown that the first plane of division
in a fertilised frog's egg is vertical and Roux established the fact
that the first plane of division is identical with the plane of symmetry
of the later embryo. Schultze found that if the frog's egg is turned
upside down at the time of its first division and kept in this abnormal
position, through compression between two glass plates for about 20
hours, a small number of eggs may give rise to twins. It is possible,
in this case, that the tendency of the black part of the egg to rotate
upwards along the surface of the egg leads to a separation of its first
cells, such a separation leading to the formation of twins.

T.H. Morgan made an interesting additional observation. He destroyed
one half of the egg after the first segmentation and found that the
half which remained alive gave rise to only one half of an embryo, thus
confirming an older observation of Roux. When, however, Morgan put the
egg upside down after the destruction of one of the first two cells, and
compressed the eggs between two glass plates, the surviving half of the
egg gave rise to a perfect embryo of half size (and not to a half embryo
of normal size as before.) Obviously in this case the tendency of the
protoplasm to flow back to its normal position was partially successful
and led to a partial or complete separation of the living from the dead
half; whereby the former was enabled to form a whole embryo, which, of
course, possessed only half the size of an embryo originating from a
whole egg.

(b) EXPERIMENTS ON HYDROIDS.

A striking influence of gravitation can be observed in a hydroid,
Antennularia antenn