acteal vessels. But such was the case. Eustachius, in
the sixteenth century, had discovered the thoracic duct in the horse,
although he seems to have thought that it was peculiar to that animal.
Aselli, while dissecting the body of a dog in 1622, accidentally
discovered the lacteals, and thought at first that they were nerves; but
upon puncturing one of them, and seeing the milky fluid which escaped,
found them to be vessels. He, however, failed to trace them to the
thoracic duct, and believed them to terminate in the liver. Pecquet of
Dieppe followed them from the intestines to the mesenteric glands, and
from these into a common sac or reservoir, which he designated
_receptaculum chyli_, and thence to their entry by a single slender
conduit into the venous system at the junction of the jugular and
subclavian veins. The existence of the lacteals had not entirely escaped
Harvey, however. He had himself noticed them in the course of his
dissections before Aselli's book was published, but "for various
reasons" could not bring himself to believe that they contained chyle.
The smallness of the thoracic duct seemed to him a difficulty, and as
it was a demonstrated fact that the gastric veins were largely
absorptive, the lacteals appeared to him superfluous. He is not
"obstinately wedded to his own opinion," and does not doubt "but that
many things, now hidden in the well of Democritus, will by-and-by be
drawn up into day by the ceaseless industry of a coming age."

Late in the author's life, as we have seen, the work on the "Generation
of Animals" appeared; but neither physiological nor microscopical
science was sufficiently advanced to admit of the production of an
enduring work on a subject necessarily so abstruse as that of
generation. It was impossible, however, for so shrewd and able an
investigator as Harvey to work at a subject even as difficult as this
without leaving the impress of his original genius. He first announced
the general truth, "Omne animal ex ovo," and clearly proved that the
essential part of the egg, that in which the reproductive processes
begin, was not the _chalazae_, but the _cicatricula_. This Fabricius had
looked upon as a blemish, a scar left by a broken peduncle. Harvey
described this little cicatricula as expanding under the influence of
incubation into a wider structure, which he called the eye of the egg,
and at the same time separating into a clear and transparent part, in
which later on, a