le-stars, holothurians or crinoids, and
still less by the sperm of more distant groups of animals. The consensus
of opinion seemed to be that the spermatozoon must enter the egg
through a narrow opening or canal, the so-called micropyle, and that
the micropyle allowed only the spermatozoa of the same or of a closely
related species to enter the egg.

It seemed to the writer that the cause of this limitation of
hybridisation might be of another kind and that by a change in the
constitution of the sea-water it might be possible to bring about
heterogenous hybridisations, which in normal sea-water are impossible.
This assumption proved correct. Sea-water has a faintly alkaline
reaction (in terms of the physical chemist its concentration of hydroxyl
ions is about (10 to the power minus six)N at Pacific Grove, California,
and about (10 to the power minus 5)N at Woods Hole, Massachusetts).
If we slightly raise the alkalinity of the sea-water by adding to it a
small but definite quantity of sodium hydroxide or some other alkali,
the eggs of the sea-urchin can be fertilised with the sperm of widely
different groups of animals, possibly with the sperm of any marine
animal which sheds it into the ocean. In 1903 it was shown that if we
add from about 0.5 to 0.8 cubic centimetre N/10 sodium hydroxide to
50 cubic centimetres of sea-water, the eggs of Strongylocentrotus
purpuratus (a sea-urchin which is found on the coast of California)
can be fertilised in large quantities by the sperm of various kinds of
starfish, brittle-stars and holothurians; while in normal sea-water or
with less sodium hydroxide not a single egg of the same female could
be fertilised with the starfish sperm which proved effective in the
hyper-alkaline sea-water. The sperm of the various forms of starfish was
not equally effective for these hybridisations; the sperm of Asterias
ochracea and A. capitata gave the best results, since it was possible to
fertilise 50 per cent or more of the sea-urchin eggs, while the sperm of
Pycnopodia and Asterina fertilised only 2 per cent of the same eggs.

Godlewski used the same method for the hybridisation of the sea-urchin
eggs with the sperm of a crinoid (Antedon rosacea). Kupelwieser
afterwards obtained results which seemed to indicate the possibility of
fertilising the eggs of Strongylocentrotus with the sperm of a mollusc
(Mytilus.) Recently, the writer succeeded in fertilising the
eggs of Strongylocentrotus franciscanu