choice or gem pearls, those beautiful symmetrical objects of great luster which are usually referred to in speaking of pearls.
Examinations
of many of these have failed, except in rare instances, to reveal a
foreign nucleus of sand or similar inorganic substance. In searching
many fresh-water mussels, Sir Everard Home frequently met with small
pearls in the ovarium, and he further noticed that these, as well as
oriental pearls, when split into halves, often showed a brilliant cell
in the center, about equal in size to the ova of the same mol-lusk.
From these observations, in 1826 he deduced his "abortive ova" theory,
and announced :
A
pearl is formed upon the external surface of an ovum, which, having
been blighted, does not pass with the others into the oviduct, but
remains attached to its pedicle in the ovarium, and in the following
season receives a coat of nacre at the same time that the internal
surface of the shell receives its annual supply. This conclusion is
verified by some pearls being spherical, others having a pyramidal
form, from the pedicle having received a coat of nacre as well as the
ovum.1
Naturalists
generally accepted these conclusions, that pearls originate in
pathological secretions formed, either as the result of the intrusion
of hard substances, or by the encysting or covering of ova or other
objects of internal origin ; and there was no important cleavage of
opinion until the development of the parasitic theory, as a result of
the researches of the Italian naturalist Filippi, and those following
his line of investigations. This theory is not severely in conflict
with those of Reaumur, Linnaeus, Home, etc., but relates principally to
the identity of the irritating or stimulating substance which forms the
nucleus of the pearl.
In examining a species of fresh-water mussel, the Anodonta cygnea, occurring
in ponds near Turin, and especially the many small pearly formations
therein, Filippi observed that these were associated with the presence
of a trematode or parasitic worm, which he named Disto-mum duplicatum, and
which appears to be closely allied to the parasite which causes the
fatal "rot" or distemper in sheep. Under the microscope, the smallest
and presumably the youngest of these pearls showed organic nuclei which
appeared undoubtedly to be the remnants of the trematode. In Anodonta
from other regions, which were not infested with the distoma, pearls
were very rarely found by Filippi. In a paper,2 published in
1852, containing a summary of his observations, he concluded that a
leading, if not the principal, cause of pearl-forma-
1 "Philosophical Transactions," 1826, Pt. Ill, ' "Sull'origine délie Perle. Il Cimento,
pp. 338-341.
revista di Scienze," Torino, 1852, Vol. I,
pp. 429-439·
