ation in both is one and identical in kind: the origin of
either is from an egg, or at least something that by analogy is
held to be so. An egg is, as already said, a conception exposed
beyond the body of the parent, whence the embryo is produced; a
conception is an egg remaining within the body of the parent until
the foetus has acquired the requisite perfection; in everything
else they agree; they are both alike primordially vegetables,
potentially they are animals.[23]

The ovum, for Harvey, is in essence "the primordium vegetable or
vegetative incipience, understanding by this a certain corporeal
something having life in potentia; or a certain something existing _per
se_, which is capable of changing into a vegetative form under the
agency of an internal principle."[24] The ovum is for Harvey more a
concept than an observed fact, and, as stated by one student of
generation, "The _dictum ex ovo omnia_, whilst substantially true in the
modern sense, is neither true nor false as employed by Harvey, since to
him it has no definite or even intelligible meaning."[25]

Harvey's treatise on generation is clearly a product of his time. It
advances embryology by its demonstration of certain facts of
development, by its aggressive espousal of epigenesis and the origin of
all animals from eggs, and by its dynamic approach stressing the
temporal factors in development and the initial independent function of
embryonic organs. However, the strong Aristotelian cast of Harvey's
treatise encouraged continued discussion of long outdated questions in
an outdated manner and, combined with his expressed disdain for
"chymistry" and atomism, discouraged close cooperation between
embryologists of different persuasions. It is perhaps easy to
underestimate the impact and general importance of Harvey's work in view
of these qualifications, and so it should be remarked that both positive
and negative features of _De Generatione_ influenced profoundly
subsequent embryological thought.

It will be recalled that the title of _The Philosophicall Touch-Stone_
identified Digby as the object of Alexander Ross's ire. In comparable
manner, the latter's _Arcana Microcosmi_, published in 1652, declares
its purpose to be "a refutation of Dr. Brown's Vulgar Errors, the Lord
Bacon's Natural History, and Dr. Harvy's book _De Generatione_." Let us
pause a brief moment in memory of a man so intrepid as to undertake the
refu